Apparatus and method for controlling an underground boring tool

ABSTRACT

An apparatus and method for determining a location and an orientation of an underground boring tool by employment of a radar-like probe and detection technique. The boring tool is provided with a device which generates a specific signature signal in response to a probe signal transmitted from above the ground. Cooperation between the probe signal transmitter at ground level and the signature signal generating device provided at the underground boring tool results in accurate detection of the boring tool location and, if desired, orientation, despite the presence of a large background signal. Precision detection of the boring tool location and orientation enables the operator to accurately locate the boring tool during operation and, if provided with a directional capacity, avoid buried obstacles such as utilities and other hazards. The signature signal produced by the boring tool may be generated either passively or actively, and may be a microwave or an acoustic signal. Further, the signature signal may be produced in a manner which differs from that used to produce the probe signal in one or more ways, including timing, frequency content, information content, or polarization.

This is a continuation of application Ser. No. 08/784,061, filed Jan.17, 1997, now U.S. Pat. No. 5,904,210 which is incorporated herein byreference, which is a Continuation-in-Part of U.S. patent applicationSer. No. 08/587,832, filed Jan. 11, 1996, now U.S. Pat. No. 5,720,354.

BACKGROUND OF THE INVENTION

The present invention relates generally to the field of trenchlessunderground boring and, more particularly, to a system and process foracquiring positional and orientation data on an underground boring tool.

Utility lines for water, electricity, gas, telephone and cabletelevision are often run underground for reasons of safety andaesthetics. In many situations, the underground utilities can be buriedin a trench which is then back-filled. Although useful in areas of newconstruction, the burial of utilities in a trench has certaindisadvantages. In areas supporting existing construction, a trench cancause serious disturbance to structures or roadways. Further, there is ahigh probability that digging a trench may damage previously buriedutilities, and that structures or roadways disturbed by digging thetrench are rarely restored to their original condition. Also, an opentrench poses a danger of injury to workers and passersby.

The general technique of boring a horizontal underground hole hasrecently been developed in order to overcome the disadvantages describedabove, as well as others unaddressed when employing conventionaltrenching techniques. In accordance with such a general horizontalboring technique, also known as microtunnelling or trenchlessunderground boring, a boring system is situated on the ground surfaceand drills a hole into the ground at an oblique angle with respect tothe ground surface. Water is typically flowed through the drill string,over the boring tool, and back up the borehole in order to removecuttings and dirt. After the boring tool reaches a desired depth, thetool is then directed along a substantially horizontal path to create ahorizontal borehole. After the desired length of borehole has beenobtained, the tool is then directed upwards to break through to thesurface. A reamer is then attached to the drill string which is pulledback through the borehole, thus reaming out the borehole to a largerdiameter. It is common to attach a utility line or other conduit to thereaming tool so that it is dragged through the borehole along with thereamer.

In order to provide for the location of a boring tool, whileunderground, a conventional approach involves the incorporation of anactive beacon, typically in the form of a radio transmitter, disposedwithin the boring tool. A receiver is typically placed on the groundsurface and used to determine the position of the tool through aconventional radio direction finding technique. However, since there isno synchronization between the beacon and the detector, the depth of thetool cannot be measured directly, and the position measurement of theboring tool is restricted to a two dimensional surface plane. Moreover,conventional radio direction finding techniques have limited accuracy indetermining the position of the boring tool. These limitations can havesevere consequences when boring a trenchless underground hole in an areawhich contains several existing underground utilities or other naturalor man-made hazards, in which case the location of the boring tool mustbe precisely determined in order to avoid accidentally disturbing ordamaging the utilities.

Recently the use of ground penetrating radar (GPR) for performingsurveys along trenchless boring routes has been proposed.Ground-penetrating-radar is a sensitive technique for detecting evensmall changes in the subsurface dielectric constant. Consequently, theimages generated by GPR systems contain a great amount of detail, muchof it either unwanted or unnecessary for the task at hand. A majordifficulty, therefore, in using GPR for locating a boring tool concernsthe present inability in the art to correctly distinguish the boringtool signal from all of the signals generated by the other features,such signals collectively being referred to as clutter. Moreover,depending on the depth of the boring tool and the propagationcharacteristics of the intervening ground medium, the signal from theboring tool can be extremely weak relative to the clutter signal.Consequently, the boring tool signal may either be misinterpreted orundetectable.

It would be desirable to employ an apparatus for determining a locationand an orientation of an underground boring tool with higher accuracythan is currently attainable given the present state of the technology.

SUMMARY OF THE INVENTION

The present invention relates to an apparatus and method for determininga location and an orientation of an underground boring tool byemployment of a radar-like probe and detection technique. The boringtool is provided with a device which generates a specific signaturesignal in response to a probe signal transmitted from above the ground.Cooperation between the probe signal transmitter at ground level and thesignature signal generating device provided at the underground boringtool results in accurate detection of the boring tool location and, ifdesired, orientation, despite the presence of a large background signal.Precision detection of the boring tool location and orientation enablesthe operator to accurately locate the boring tool during operation and,if provided with a directional capacity, avoid buried obstacles such asutilities and other hazards.

The signature signal produced by the boring tool may be generated eitherpassively or actively, and may be a microwave or an acoustic signal.Further, the signature signal may be produced in a manner which differsfrom that used to produce the probe signal in one or more ways,including timing, frequency content, information content, orpolarization.

In accordance with one embodiment, surveying the boring site, eitherprior to or during the boring operation, provides for the production ofdata associated with the characteristics of the ground medium subjectedto the survey. The ground characteristic data acquired during the surveymay be correlated with historical data which relate ground types toboring productivity, hence enabling estimates of boring productivity andoverall cost to be made for the site subjected to the survey. Accuratesurveys of planned boring pathways can be made and the position of theboring tool accurately measured during a boring operation forcontemporaneous or subsequent comparison with the planned pathway. Thedirection of the boring tool may be adjusted in response to the measuredposition in order to maintain the boring tool along the planned pathway.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a trenchless underground boring apparatus inaccordance with an embodiment of the present invention;

FIG. 2 is a detailed schematic side view of the trenchless undergroundboring tool and a probe and detection unit shown in FIG. 1;

FIG. 3 is a graph depicting time domain signature signal generation;

FIG. 4 is a graph depicting frequency domain signature signalgeneration;

FIGS. 5a-5c show three embodiments for passive microwave signaturesignal generation;

FIGS. 6a-6d show four embodiments for active microwave signature signalgeneration;

FIGS. 7a-7b show two embodiments for active acoustic signature signalgeneration;

FIG. 8 shows an embodiment of a cooperative target incorporating asignature signal generator and an orientation detector;

FIG. 9 is an illustration of an orientation detector for detecting anorientation of a cooperative target;

FIG. 10 is a block diagram of an orientation detector which, inaccordance with one embodiment, detects an orientation of a cooperativetarget and produces an output indicative of such orientation, and, inaccordance with another embodiment, produces an output signature signalthat indicates both a location and an orientation of the undergroundboring tool;

FIGS. 11a-11b illustrate an embodiment of an orientation detectingapparatus which includes a number of passive signature signal generatingdevices that provide both boring tool location and orientationinformation;

FIG. 12 illustrates another embodiment of an orientation detector thatproduces an output indicative of an orientation of the undergroundboring tool;

FIGS. 13a-13b illustrate another embodiment of a passive orientationdetector that produces a signature signal indicative of both thelocation and orientation of the underground boring tool;

FIG. 14 illustrates an embodiment of an orientation detector suitablefor incorporation in an underground boring tool that produces an outputindicative of the rotational orientation and pitch of the boring tool;

FIG. 15 shows an embodiment of a boring tool incorporating an activesignature signal generator and an orientation detection apparatus;

FIG. 16 is a diagram of a methodology for determining the depth of anunderground boring tool incorporating a cooperative target by use of atleast two receive antennas and a single transmit antenna;

FIG. 17a is a depiction of an underground boring tool trackingmethodology using an array of two receive antennas and a transmitantenna provided within the receive antenna array;

FIG. 17b is a graph illustrating signature signal detection by each ofthe antennae in the receive antenna array of FIG. 17a which, in turn, isused to determine a location and deviation of an underground boring toolrelative to a predetermined above-ground path;

FIG. 18a is a depiction of an underground boring tool trackingmethodology using an array of four receive antennas and a transmitantenna provided within the receive antenna array;

FIG. 18b is a graph illustrating signature signal detection by each ofthe four antennae in the receive antenna array of FIG. 18a which, inturn, is used to determine a location and deviation of an undergroundboring tool relative to a predetermined above-ground path;

FIG. 19 is an illustration of a single-axis antenna system typicallyused with a ground penetrating radar system for providingtwo-dimensional subsurface geologic imaging;

FIG. 20 is an illustration of an antenna system including a plurality ofantennae oriented in an orthogonal relationship for use with a groundpenetrating radar system to provide three-dimensional subsurfacegeologic imaging in accordance with one embodiment of the invention;

FIGS. 21a-d illustrate an embodiment of a trenchless underground boringtool incorporating various sensors, and further depicts sensor signalinformation;

FIGS. 22a-d illustrate an embodiment of a trenchless underground boringtool incorporating an active beacon and various sensors, and furtherdepicts sensor signal information;

FIG. 23 is an illustration of a boring site having a heterogeneoussubsurface geology;

FIG. 24 is a system block diagram of a trenchless boring system controlunit incorporating position indicators, a geographical recording system,various databases, and a geological data acquisition unit;

FIG. 25 is an illustration of a boring site and a trenchless boringsystem incorporating position location devices;

FIG. 26 illustrates in flow diagram form generalized method steps forperforming a pre-bore survey;

FIG. 27 is a system block diagram of a trenchless underground boringsystem control unit for controlling the boring operation; and

FIGS. 28-29 illustrate in flow diagram form generalized method steps forperforming a trenchless boring operation.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring now to the figures and, more particularly, to FIG. 1, there isillustrated an embodiment of a trenchless underground boring systemincorporating a detection system for detecting a location and anorientation of an underground boring tool. The detection system includesan above-ground probing and detection unit 28 (PDU) and a below-groundcooperative target 20 mounted to, contained in, or otherwise coupled toan underground boring tool 24.

The PDU 28 and the target 20 operate in cooperation to provide reliableand accurate locating of an underground boring tool 24. In addition, theorientation of the boring tool 24 during operation may also be provided.In terms of general operation, the PDU 28 transmits a probe signal 36into the ground 10 and detects return signals reflected from the groundmedium, and the underground boring tool 24. The return signals typicallyincludes content from many different reflection sources, often renderingdetection of the underground boring tool 24 unreliable or impossibleusing conventional techniques. Detecting an underground boring tool 24is greatly enhanced by use of the cooperative target 20, which, inresponse to the probe signal 36, emits a signature signal that isreadily distinguishable from the return signals reflected by the groundmedium and the underground boring tool 24. The cooperative target 20 mayalso include an orientation detection apparatus that senses anorientation of the boring tool 24. Boring tool orientation informationmay be transmitted with the location information as a compositesignature signal or as an information signal separate from the signaturesignal. As such, the presence, location, and orientation of anunderground boring tool 24 is readily and reliably determined byemploying the probing and detection system and method of the presentinvention.

It is well known in the field of subsurface imaging that conventionalunderground imaging techniques, such as those that employ GPR, detectthe presence of many types of underground obstructions and structures.It is also well known in the art that detecting objects of interest,such as an underground boring tool 24, is often made difficult orimpossible due to the detection of return signals emanating from manysources not of interest, collectively known as clutter, associated withother underground obstructions, structures, and varying ground mediumcharacteristics, for example. The clutter signal represents backgroundnoise in the composite return signal above which a return signal ofinterest must be distinguished. Attempting to detect the presence of theunderground boring tool 24 using a conventional approach often rendersthe boring tool 24 undetectable or indistinguishable from the backgroundnoise.

It is understood that the return signal from an underground object ofinterest using conventional detection techniques may be weak relative tothe clutter signal content. In such a case, the signal-to-clutter ratiowould be low, which reduces the ability to clearly detect the returnsignal emanating from the underground object of interest. The probe anddetection apparatus and method of the present invention advantageouslyprovides for the production of a return signal from the cooperativetarget 20 provided at the underground boring tool 24 having acharacteristic signature which can be more easily distinguished from theclutter. As will be discussed in detail hereinbelow, the generation of asignature signal containing either or both location and orientationinformation by the cooperative target 20 may be performed eitherpassively or actively.

FIG. 1 illustrates a cross-section through a portion of ground 10 wherethe boring operation takes place, with most of the components of thedetection system depicted situated above the ground surface 11. Thetrenchless underground boring system, generally shown as the system 12,includes a platform 14 on which is situated a tilted longitudinal member16. The platform 14 is secured to the ground by pins 18 or otherrestraining members in order to prevent the platform 14 from movingduring the boring operation. Located on the longitudinal member 16 is athrust/pullback pump 17 for driving a drill string 22 in a forward,longitudinal direction as generally shown by the arrow. The drill string22 is made up of a number of drill string members 23 attachedend-to-end. Also located on the tilted longitudinal member 16, andmounted to permit movement along the longitudinal member 16, is arotating motor 19 for rotating the drill string 22 (illustrated in anintermediate position between an upper position 19a and a lower position19b). In operation, the rotating motor 19 rotates the drill string 22which has a boring tool 24 at the end of the drill string 22.

A typical boring operation takes place as follows. The rotating motor 19is initially positioned in an upper location 19a and rotates the drillstring 22. While the boring tool 24 is rotated, the rotating motor 19and drill string 17 are pushed in a forward direction by thethrust-pullback pump 20 toward a lower position into the ground, thuscreating a borehole 26. The rotating motor 19 reaches a lower position19b when the drill string 22 has been pushed into the borehole 26 by thelength of one drill string member 23. A new drill string member 23 isthen added to the drill string 22 either manually or automatically, andthe rotating motor 19 is released and pulled back to the upper location19a. The rotating motor 19 then clamps on to the new drill string member23 and the rotation/push process is repeated so as to force the newlylengthened drill string 22 further into the ground, thereby extendingthe borehole 26. Commonly, water is pumped through the drill string 22and back up through the borehole to remove cuttings, dirt, and otherdebris. If the boring tool 24 incorporates a directional steeringcapability for controlling its direction, a desired direction can beimparted to the resulting borehole 26.

In FIG. 1, there is illustrated a borehole 26 which bends in thevicinity of a point 31 after the initial oblique section becomesparallel to the ground surface 11. Located above the surface 11, anddetachable from the trenchless underground boring system 12, is aprobing and detection unit 28 (PDU), mounted on wheels 29 or tracks inorder to permit above-ground traversing of the PDU 28 along a pathcorresponding to the underground path of the boring tool 24. The PDU 28is coupled to a control unit 32 via a data transmission link 34.

The operation of the PDU 28 is more clearly described in reference toFIG. 2. The PDU 28 is generally used to transmit a probe signal 36 intothe ground and to detect returning signals. The PDU 28 contains agenerator 52 for generating the probe signal 36 which probes the ground10. A transmitter 54 receives the probe signal 36 from the generator 52,which, in turn, transmits the probe signal 36 (shown as continuous linesin FIG. 2) into the ground 10. In a first embodiment, the generator 52is a microwave generator and the transmitter 54 is a microwave antennafor transmitting microwave probe signals. In an alternative embodiment,the generator 52 is an acoustic wave generator and produces acousticwaves, and the transmitter 54 is typically a probe placed into theground 10 to provide for good mechanical contact for transmitting theacoustic waves into the ground 10.

The probe signal 36 is transmitted by the PDU 28, propagates through theground 10, and encounters subsurface obstructions, one of which is shownas 30, which scatter a return signal 40 (shown as dotted lines in FIG.2) back to the PDU 28. A signature signal 38 (shown as dashed lines inFIG. 2) is also returned to the PDU 28 from the boring tool 24 locatedin the borehole 26.

The detection section of the PDU 28 includes a receiver 56, a detector58, and a signal processor 60. The receiver 56 receives the returnsignals from the ground 10 and communicates them to the detector 58. Thedetector 58 converts the return signals into electric signals which aresubsequently analyzed in the signal processing unit 60. In the firstembodiment described hereinabove in which the probe signal 36constitutes a microwave signal, the receiver 56 typically includes anantenna, and the detector 58 typically includes a detection diode. Inanother embodiment in which the probe signal 36 constitutes an acousticwave, the receiver 56 typically is a probe in good mechanical contactwith the ground 10 and the detector 58 includes a sound-to-electricaltransducer, such as microphone. The signal processor 60 may includevarious preliminary components, such as a signal amplifier, a filteringcircuit, and an analog-to-digital converter, followed by more complexcircuitry for producing a two or three dimensional image of a subsurfacevolume which incorporates the various underground obstructions 30 andthe boring tool 24. The PDU 28 also contains a beacon receiver/analyzer61 for detecting and interpreting a signal from an underground activebeacon. The function of the beacon receiver/analyzer 61 will bedescribed more fully hereinbelow.

The PDU 28 also contains a decoder 63 for decoding information signalcontent that may be encoded on the signature signal produced by thecooperative target 20. Orientation, pressure, and temperatureinformation, for example, may be sensed by appropriate sensors providedin the cooperative target 20, such as a strain gauge for sensingpressure. Such information may be encoded on the signature signal, suchas by modulating the signature signal with an information signal, orotherwise transmitted as part of, or separate from, the signaturesignal. When received by the receiver 56, an encoded return signal isdecoded by the decoder 61 to extract the information signal content fromthe signature signal content. It is noted that the components of the PDU28 illustrated in FIG. 2 need not be contained within the same housingor supporting structure.

Referring once again to FIG. 1, the PDU 28 transmits acquiredinformation along the data transmission link 34 to the control unit 32,which is illustrated as being located in proximity to the trenchlessunderground boring system 12. The data transmission link 34 is providedto handle the transfer of data between the PDU 28 and the trenchlessunderground boring system 12, and may be a co-axial cable, an opticalfiber, a free-space link for infrared communication, or some othersuitable data transfer medium or technique. A significant advantage ofusing a trenchless underground boring system 12 which employs thesubsurface detection technique described herein concerns the detectionof other important subsurface features which may purposefully be avoidedby the boring tool 24, particularly buried utilities such as electric,water, gas, sewer, telephone lines, cable lines, and the like.

Signature signal generation, in accordance with the embodiments of FIGS.3 and 4, may be accomplished using temporal and frequency basedtechniques, respectively. FIG. 3 is an illustration depicting thegeneration and detection of an underground boring tool signature signalin the time domain. Line A shows the emission of a probe signal 36a as afunction of signal character plotted against time. Line B shows a returnsignal 62a detected by the PDU 28 in the absence of any signature signalgeneration. The return signal 62a is depictive of a signal received bythe PDU 28 at a time ΔT1 after emission of the probe signal 36a, and isrepresented as a commixture of signals returned from the undergroundstructure 22 and other scatterers. As previously discussed, a lowsignal-to-clutter ratio makes it very difficult to distinguish thereturn signal from the underground boring tool 24.

Line C illustrates an advantageous detection technique in whichcooperation between the cooperative target 22, provided at the boringtool 24, and the PDU 28 is employed to produce and transmit a signaturesignal at a certain time ΔT2 following illumination with the probesignal 36a. In accordance with this detection scheme, the return signal40a received from the scatterers is detected initially, and thesignature signal 38a received from the underground boring tool 24 isdetected after a delay of ΔT2. The delay time ΔT2 is established to besufficiently long so that the signature signal produced by thecooperative target 20 is significantly more pronounced than the cluttersignal at the time of detection. In this case, the signal-to-clutterratio of the signature signal 38a is relatively high, thus enabling thesignature signal 38a to be easily distinguished from the backgroundclutter 40a.

FIG. 4 is an illustration depicting the detection of a cooperativetarget signature signal emitted from an underground boring tool 24 inthe frequency domain. Line A illustrates the frequency band 36b of theprobe signal as a function of signal strength plotted against frequency.Line B shows a frequency band 62b of a return signal received from theunderground boring tool 24 in the absence of any cooperative signalgeneration. It can be seen that the naturally occurring return signalsfrom the underground boring tool 24 and other scatterers 30 share afrequency band 62b similar to that of the probe signal 36b. Line Cillustrates a case where cooperation is employed between the cooperativetarget 20 of the underground boring tool 24 and the PDU 28 to produceand transmit a signature signal which has a frequency band 38b differentfrom that of the scattered return signal 40b. The difference infrequency band, indicated as Δf, is sufficiently large to move thecooperative target signature signal out of, or at least partiallybeyond, the scattered signal frequency band 40b. Thus, the cooperativetarget signature signal can be detected with relative ease due to theincreased signal-to-clutter ratio. It is noted that high pass, low pass,and notch filtering techniques, for example, or other filtering andsignal processing methods may be employed to enhance cooperative targetsignature signal detection.

It is an important feature of the invention that the underground boringtool 24 be provided with a signature signal-generating apparatus, suchas a cooperative target 20, which produces a signature signal inresponse to a probe signal transmitted by the PDU 28. If no suchsignature signal was produced by the generating apparatus, the PDU 28would receive an echo from the underground boring tool 24 which would bevery difficult to distinguish from the clutter with a high degree ofcertainty using conventional detecting techniques. The incorporation ofa signature signal generating apparatus advantageously provides for theproduction of a unique signal by the underground boring tool 24 that iseasily distinguishable from the clutter and has a relatively highsignal-to-clutter ratio. As discussed briefly above, an active orpassive approach is suitable for generating the boring tool signaturesignal. It is understood that an active signature signal circuit is onein which the circuit used to generate the signature signal requires theapplication of electrical power from an external source, such as abattery, to make it operable. A passive circuit, in contrast, is onewhich does not utilize an external source of power. The source of energyfor the electrical signals present in a passive circuit is the receivedprobe signal itself.

In accordance with a passive approach, the cooperative target 20 doesnot include an active apparatus for generating or amplifying a signal,and is therefore generally less complex than an active approach since itdoes not require the presence of a permanent or replaceable power sourceor, in many cases, electronic circuitry. Alternatively, an activeapproach may be employed which has the advantage that it is moreflexible and provides the opportunity to produce a wider range ofsignature response signals which may be more identifiable whenencountering different types of ground medium. Further, an activeapproach reduces the complexity and cost of manufacturing thecooperative target 20, and may reduce the complexity and cost of thesignature signal receiving apparatus.

Three embodiments of a passive signature signal generating apparatusassociated with a microwave detection technique are illustrated in FIG.5. Each of the embodiment illustrations shown in FIG. 5 includes aschematic of a cooperative target 20 including a microwave antenna andcircuit components which are used to generate the signature signal. Thethree embodiments illustrated in FIGS. 5a, 5b, and 5c are directedtoward the generation of the signature signal using a) the time domain,b) the frequency domain and c) cross-polarization, respectively.

In FIG. 5a, there is illustrated a cooperative target 20 which includestwo antennae, a probe signal receive antenna 66a, and a signature signaltransmit antenna 68a. For purposes of illustration, these antennae areillustrated as separate elements, but it is understood that microwavetransmit/receive systems can operate using a single antenna for bothreception and transmission. Two separate antennae are used in theillustration of this and the following embodiments in order to enhancethe understanding of the invention and, as such, no limitation of theinvention is to be inferred therefrom. The receive antenna 66a and thetransmit antenna 68a in the physical embodiment of the signature signalgenerator will preferably be located inside the cooperative target 20 oron its surface in a conformal configuration. For antennae locatedentirely within the cooperative target 20, it is understood that atleast a portion of the cooperative target housing is made of anon-metallic material, preferably a hard dielectric material, thusallowing passage of the microwaves through at least a portion of thecooperative target housing. A material suitable for this application isKEVLAR®. Antennae that extend outside of the cooperative target housingmay be covered by a protective non-metallic material. The antennae, inthis configuration, may be made to conform to the housing contour, ordisposed in recesses provided in the housing and covered with an epoxymaterial, for example.

The illustration of FIG. 5a shows the signature signal generationapparatus for a microwave detection system operating in the time domain.In accordance with this embodiment, a receive antenna 66a receives aprobe signal 70a from the PDU 28, such as a short microwave burstlasting a few nanoseconds, for example. In order to distinguish asignature signal 74a from the clutter received by the PDU 28, thereceived probe signal 70a passes from the receive antenna 66a into atime-delaying waveguide 72a, preferably a co-axial cable, to a transmitantenna 68a. The signature signal 74a is then radiated from the transmitantenna 68a and received by the PDU 28. The use of the time-delay line,which preferably delays the response from the cooperative target 20 byabout 10 nanoseconds, delays radiating the return signature signal 74auntil after the clutter signal received by the PDU 28 has decreased inmagnitude.

In accordance with another embodiment, a single antenna embodiment ofthe passive time domain signature generator could be implemented bycutting the waveguide at the point indicated by the dotted line 76a toform a termination. In this latter embodiment, the probe signal 70apropagates along the waveguide 72a until it is reflected by thetermination located at the cut 76a, propagates back to the receiveantenna 66a, and is transmitted back to the PDU 28. The terminationcould be implemented either as an electrical short, in which case theprobe signal 70a would be inverted upon reflection, or as an opencircuit, in which case the probe signal 70a would not be inverted uponreflection.

The introduction of a time delay to create the signature signal 74amakes the underground boring tool 24 appear deeper in the ground than itis in actuality. Since microwaves are heavily attenuated by the ground,ground penetrating radar systems have a typical effective depth range ofabout 10 feet when employing conventional detection techniques, beyondwhich point the signal returns are generally too heavily attenuated tobe reliably detected. The production of a time delayed signature signalreturn 74a from the underground boring tool 24 artificially translatesthe depth of the underground boring tool 24 to an apparent depth in therange of 10 to 20 feet, a depth from which there is generally no otherstrong signal return, thus significantly enhancing the signal-to-clutterratio of the detected signature signal 74a. The actual depth of theunderground boring tool 24 may then be determined by factoring out theartificial depth component due to the known time delay associated withthe cooperative target 20. It is believed that the signature signalgenerated by a cooperative target 20 may be detectable at actual depthson the order of 100 feet. It is further believed that a signature signalgenerated by an active device will generally be stronger, and thereforemore detectable, than a signature signal produced by a passive device.

The illustration of FIG. 5b depicts a signature signal generatingapparatus for a microwave detection system operating in the frequencydomain. In accordance with this embodiment, a receive antenna 66b,provided in or on the boring tool 24, receives a microwave probe signal70b from the PDU 28. The probe signal 70b is preferably a microwaveburst, lasting for several microseconds, which is centered on a givenfrequency, f, and has a bandwidth of Δf1, where Δf1/f is typically lessthan one percent. In order to shift a return signature signal 74b out ofthe frequency regime associated with the clutter received by the PDU 28,the received probe signal 70b propagates from the receive antenna 66balong a waveguide 72b into a nonlinear device 78b, preferably a diode,which generates harmonic signals, such as second and third harmonics,from an original signal.

The harmonic signal is then radiated from a transmit antenna 68b as thesignature signal 74b and is received by the PDU 28. The PDU 28 is tunedto detect a harmonic frequency of the probe signal 70b. For a probesignal 70b of 100 MHz, for example, a second harmonic detector 58 wouldbe tuned to 200 MHz. Generally, scatterers are linear in their responsebehavior and generate a clutter signal only at a frequency equal to thatof the probe signal 70b. Since there is generally no other source of theharmonic frequency present, the signal-to-clutter ratio of the signaturesignal 74b at the harmonic frequency is relatively high. In a mannersimilar to that discussed hereinabove with respect to the passive timedomain embodiment, the passive frequency domain embodiment may beimplemented using a single antenna by cutting the waveguide at the pointindicated by the dotted line 76b to form a termination. In accordancewith this latter embodiment, the probe signal 70b would propagate alongthe waveguide 72b, through the nonlinear element 78b, reflect at thetermination 76b, propagate back through the nonlinear element 78b,propagate back to the receive antenna 66b, and be transmitted back tothe PDU 28. The polarity of the reflection would be determined by thenature of the termination, as discussed hereinabove.

The illustration of FIG. 5c depicts signature signal generation for amicrowave detection system operating in a cross-polarization mode. Inaccordance with this embodiment, the PDU 28 generates a probe signal 70cof a specific linear polarity which is then transmitted into the ground.The clutter signal is made up of signal returns from scatterers which,in general, maintain the same polarization as that of the probe signal70c. Thus, the clutter signal has essentially the same polarization asthe probe signal 70c. A signature signal 74c is generated in thecooperative target 20 by receiving the polarized probe signal 70c in areceive antenna 66c, propagating the signal through a waveguide 72c to atransmit antenna 68c, and transmitting the signature signal 74c back tothe PDU 28. The transmit antenna 68c is oriented so that thepolarization of the radiated signature signal 74c is orthogonal to thatof the received probe signal 70c. The PDU 28 may also be configured topreferentially receive a signal whose polarization is orthogonal to thatof the probe signal 70c. As such, the receiver 56 preferentially detectsthe signature signal 74c over the clutter signal, thus improving thesignature signal-to-clutter ratio.

In a manner similar to that discussed hereinabove with respect to thepassive time and frequency domain embodiments, the cross-polarizationmode embodiment may be implemented using a single antenna by cutting thewaveguide at the point indicated by the dotted line 76c to form atermination and inserting a polarization mixer 78c which alters thepolarization of the wave passing therethrough. In this latterembodiment, the probe signal would propagate along the waveguide 72c,through the polarization mixer 78c, reflect at the termination 76c,propagate back through the polarization mixer 78c, propagate back to thereceive antenna 66c and be transmitted back to the PDU 28. The polarityof the reflection may be determined by the nature of the termination, asdiscussed previously hereinabove. It is understood that an antennaemployed in the single antenna embodiment would be required to haveefficient radiation characteristics for orthogonal polarizations. It isfurther understood that the cross-polarization embodiment may employcircularly or elliptically polarized microwave radiation. It is alsounderstood that the cross-polarization embodiment may be used in concertwith either the passive time domain or passive frequency domainsignature generation embodiments described previously with reference toFIGS. 5a and 5b in order to further enhance the signal-to-clutter ratioof the detected signature signal.

Referring now to FIG. 6, active signature signal generation embodimentswill be described. FIG. 6a illustrates an embodiment of active timedomain signature signal generation suitable for incorporation in aboring tool 24. The embodiment illustrated shows a probe signal 82abeing received by a receive antenna 84a which is coupled to a delay-linewaveguide 86a. An amplifier 88a is located at a point along thewaveguide 86a, and amplifies the probe signal 82a as it propagates alongthe waveguide 86a. The amplified probe signal continues along thedelay-line waveguide 86a to the transmit antenna 90a which, in turn,transmits the signature signal 92a back to the PDU 28. FIG. 6billustrates an alternative embodiment of the active time domainsignature generator which incorporates a triggerable delay circuit forproducing the time-delay, rather than propagating a signal along alength of time-delay waveguide. The embodiment illustrated shows a probesignal 82b being received by a receive antenna 84b coupled to awaveguide 86b. A triggerable delay circuit 88b is located at a pointalong the waveguide 86b. The triggerable delay circuit 88b operates inthe following fashion. The triggerable delay circuit 88b is triggered bythe probe signal 82b which, upon initial detection of the probe signal82b, initiates an internal timer circuit. Once the timer circuit hasreached a predetermined delay time, preferably in the range 1-20nanoseconds, the timer circuit generates an output signal from thetriggerable delay circuit 88b which is used as a signature signal 92b.The signature signal 92b propagates along the waveguide 86b to atransmit antenna 90b which then transmits the signature signal 92b tothe PDU 28.

FIG. 6c illustrates an embodiment of an active frequency domainsignature generator suitable for incorporation in or on an undergroundboring tool 24. The embodiment illustrated shows a probe signal 82cbeing received by a receive antenna 84c coupled to a waveguide 86c and anonlinear element 88c. The frequency-shifted signal generated by thenonlinear element 88c is then passed through an amplifier 94c beforebeing passed to the transmit antenna 90c, which transmits the signaturesignal 92c to the PDU 28. The amplifier 94c may also include a filteringcircuit to produce a filtered signature signal at the output of theamplifier 94c. An advantage to using an active frequency domainsignature signal generation embodiment over a passive frequency domainsignature signal generation embodiment is that the active embodimentproduces a stronger signature signal which is more easily detected.

In a second embodiment of the active frequency domain signature signalgenerator, generally illustrated in FIG. 6c, a probe signal 82c passesthrough the amplifier 94c prior to reaching the nonlinear element 88c.An advantage of this alternative embodiment is that, since theamplification process may take place at a lower frequency, the amplifiermay be less expensive to implement.

A third embodiment of an active frequency domain signature generatorsuitable for use with an underground boring tool 24 is illustrated inFIG. 6d. FIG. 6d shows a receive antenna 84d coupled through use of awaveguide 86d to a frequency shifter 88d and a transmit antenna 90d. Thefrequency shifter 88d is a device which produces an output signal 92dhaving a frequency of f2, which is different from the frequency, f1, ofan input signal 82d by an offset Δf, where f2=f1+Δf. In accordance withthis embodiment, Δf is preferably larger than one half of the bandwidthof the probe signal 82d, typically on the order of 1 MHz. The frequencyshifter 88d produces a frequency shift sufficient to move the signaturesignal 92d out of, or at least partially beyond, the frequency band ofthe clutter signal, thereby increasing the signal-to-clutter ratio ofthe detected signature signal 92d. For purposes of describing theseembodiments, the term signature signal embraces all generated returnsignals from the cooperative target 20 other than those solely due tothe natural reflection of the probe signal off of the underground boringtool 24.

FIG. 7 illustrates an embodiment of a signature signal generator adaptedfor use in a cooperative target 20 provided on or within an undergroundboring tool 24 where the probe signal is an acoustic signal. In anacoustic time-domain embodiment, as illustrated in FIG. 7a, an acousticprobe signal 98a, preferably an acoustic impulse, is received anddetected by an acoustic receiver 100a mounted on the inner wall 96a ofthe boring tool 24. The acoustic receiver 100a transmits a triggersignal along a trigger line 102a to a delay pulse generator 104a. Afterbeing triggered, the delay pulse generator 104a generates a signaturepulse following a triggered delay. The signature pulse is passed alongthe transmitting line 106a to an acoustic transmitter 108a, also mountedon the inner wall 96a of the boring tool 24. The acoustic transmitter108a then transmits an acoustic signature signal 110a through the groundfor detection by the PDU 28.

In accordance with an acoustic frequency-domain embodiment, as isillustrated in FIG. 7b, an acoustic probe signal 98b, preferably anacoustic pulse having a given acoustic frequency f3, is received anddetected by an acoustic receiver 100b mounted on the inner wall 96b ofthe boring tool 24. The acoustic receiver 100b transmits an inputelectrical signal corresponding to the received acoustic signal 98b at afrequency f3 along a receive line 102b to a frequency shifter 104b. Thefrequency shifter 104b generates an output electrical signal having afrequency that is shifted by an amount Δf3 relative to the frequency ofthe input signal 98b. The output signal from the frequency shifter 104bis passed along a transmit line 106b to an acoustic transmitter 108b,also mounted on the inner wall 96b of the boring tool 24. The acoustictransmitter 108b then transmits the frequency shifted acoustic signaturesignal 110b through the ground for detection by the PDU 28.

In FIG. 8, there is illustrated in system block diagram form anotherapparatus for actively generating in a cooperative target 20 a signaturesignal that contains various types of information content. In oneconfiguration, the signature signal generating apparatus of thecooperative target 20 includes a receive antenna 41, a signature signalgenerator 43, and a transmit antenna 45. In accordance with thisconfiguration, a probe signal 37 produced by the PDU 28 is received bythe receive antenna 41 and transmitted to a signature signal generator43. The signature signal generator 43 alters the received probed signal37 so as to produce a signature signal that, when transmitted by thetransmit antenna 45, is readily distinguishable from other return andclutter signals received by the PDU 28. Alternatively, the signaturesignal generator 43, in response to the received probe signal 37,generates a signature signal different in character than the receivedprobe signal 37. The signature signal transmitted by the transmitantenna 45 differs from the received probe signal 37 in one or morecharacteristics so as to be readily distinguishable from other returnand clutter signals. By way of example, and as discussed in detailhereinabove, the signature signal produced by the signature signalgenerator 43 may differ in phase, frequency content, polarization, orinformation content with respect to other return and clutter signalsreceived by the PDU 28.

Additionally, as is further illustrated in FIG. 8, the cooperativetarget 20 may include an orientation detector 47. The orientationdetector 47 is a device capable of sensing an orientation of thecooperative target 20, and provides an indication of the orientation ofthe underground boring tool 24 during operation.

It may be desirable for the operator to know the orientation of theboring tool 24 when adjusting the direction of the boring tool 24 alongan underground pathway, since several techniques known in the art fordirecting boring tools rely on a preferential orientation of the tool.If the boring tool 24 orientation is not known, the boring tool 24cannot be steered in a preferred direction in accordance with such knowntechniques that require knowledge of boring tool 24 orientation. It maynot be possible to determine the orientation of the boring tool 24simply from a knowledge of the orientation of the members 23 of thedrill string 22, since one or more members 23 of the drill string 22 maytwist or slip relative to one another during the boring operation. Sincethe boring operation takes place underground, the operator has no way ofdetecting whether such twisting or slipping has occurred. It may,therefore, be important to determine the orientation of the boring tool24.

The orientation detector 47 produces an orientation signal which iscommunicated to an encoder 49, such as a signal summing device, whichencodes the orientation signal produced by the orientation detector 47on the signature signal produced by the signature signal generator 43.

The encoded signature signal produced at the output of the encoder 49 iscommunicated to the transmit antenna 45 which, in turn, transmits theencoded signature signal 39 to the PDU 28. Various known techniques forencoding the orientation signal on the signature signal may beimplemented by the encoder 49, such as by modulating the signaturesignal with the orientation signal. It is noted that other sensors maybe included within the apparatus illustrated in FIG. 8 such as, forexample, a temperature sensor or a pressure sensor. The outputs of suchsensors may be communicated to the encoder 49 and similarly encoded onthe signature signal for transmission to the PDU 28 or, alternatively,may be transmitted as information signals independent from the signaturesignal.

Referring to FIG. 9, there is illustrated an embodiment of anorientation detecting apparatus which may include up to three mutuallyorthogonally arranged orientation detectors. The orientation detectors210, 212, and 214 are aligned along the x-axis, y-axis, and z-axis,respectively. In accordance with this embodiment, the orientationdetector 210 detects changes in orientation with respect to the x-axis,while the orientation detector 212 senses changes in orientation withrespect to the y-axis. Similarly, the orientation detector 214 detectschanges in orientation with respect to the z-axis. Given thisarrangement, changes in pitch, yaw, and roll may be detected when thecooperative target 20 is subject to positional changes. It is noted thata single orientation detector, such as detector 210, may be used tosense changes along a single axis, such as pitch changes in the boringtool 24, if multiple axis orientation changes need not be detected.Further, depending on the initial orientation of the cooperative target20 when mounted to the underground boring tool 24, two orthogonallyarranged orientation detectors, such as orientation detectors 210 and212 aligned respectively along the x and y-axes, may be sufficient toprovide pitch, yaw, and roll information.

Referring now to FIG. 10, there is illustrated an embodiment of anapparatus for detecting an orientation of an underground boring tool 24.In accordance with this embodiment, the cooperative target 20 providedon or within the underground boring tool 24 includes a tilt detector 290that detects changes in boring tool orientation during boring activity.The cooperative target 20, in addition to producing a signature signalfor purposes of determining boring tool location, may include anorientation detector, such as that illustrated in FIG. 10, for purposesof producing and orientation signal representative of an orientation ofthe cooperative target 20 and, therefore, the underground boring tool24.

In one embodiment, as is illustrated in FIG. 8, the cooperative target20 includes an orientation detecting apparatus, which produces anorientation signal, and a separate signature signal generator, whichproduces a signature signal. The signature signal and the orientationsignal may be transmitted by the transmit antenna 45 of the cooperativetarget 20 as two separate information signals or, alternatively, as acomposite signal which includes both the signature and orientationsignals. Alternatively, the orientation detecting apparatus may producea single signature signal that is indicative of both the location andthe orientation of the cooperative target 20.

Referring in greater detail to FIG. 10, there is illustrated a tiltdetector 290 coupled to a selector 291. The tilt detector 290 detectstilting of the cooperative target 20 with respect to one or moremutually orthogonal axes of the boring tool 24. It is believed that thetilt detector 290 illustrated in FIG. 10 is useful as a sensor thatsenses the pitch of the boring tool 24 during operation. The range oftilt angles detectable by the tilt detector 290 may be selected inaccordance with the estimated amount of expected boring tool tilting fora given application. For example, the tilt detector 290 may detectmaximum pitch angles in the range of ±45° relative to horizontal in oneapplication, whereas, in another application, the tilt detector 290 maydetect pitch angles in the range of ±90° relative to horizontal, forexample. It is to be understood that the tilt detector 290, as well asother components illustrated in FIG. 10, may be active or passivecomponents.

As is further illustrated in FIG. 10, a probe signal 235 is received bythe receive antenna 234 which, in turn, communicates the probe signal235 to a selector 291. The tilt detector 290 and selector 291 cooperateto select one of several orientation signal generators depending on themagnitude of tilting as detected by the tilt detector 290. In oneembodiment, the probe signal 235 is coupled to each of the orientationsignal generators P₁ 292 through P_(N) 297, one of which is selectivelyactivated by the tilt detector 290 which incorporates the function ofthe selector 291, such as the embodiment illustrated in FIG. 12. Inanother embodiment, the probe signal 235 is coupled to the selector 291which activates one of the orientation signal generators P₁ 292 throughP_(N) 297 depending on the magnitude of tilting detected by the tiltdetector 290.

By way of example, and in accordance with a passive componentimplementation, each of the orientation signal generators P₁ 292 throughP_(N) 297 represent individual transmission lines, each of whichproduces a unique time-delayed signature signal which, when transmittedby the transmit antenna 244, provides both location and orientationinformation when received by the PDU 28. As such, the orientationdetection apparatus in accordance with this embodiment provides bothlocation and orientation information and does not require a separatesignature signal generator 43. In another embodiment, each of theorientation signal generators, such as orientation signal generator P₃294, produces a unique orientation signal which is transmitted to anencoder 49. A signature signal 299 produced by a signature signalgenerator 43 separate from the orientation detection apparatus may beinput to the encoder 49, which, in turn, produces a composite signaturesignal 301 which includes both signature signal and orientation signalcontent. The composite signal 301 is then transmitted to the PDU 28 anddecoded to extract the orientation signal content from the signaturesignal content.

As discussed previously, the range of tilt angles detectable by the tiltdetector 290 and the resolution between tilt angle increments may varydepending on a particular application or use. By way of example, it isassumed that the tilt detector 290 is capable of detecting maximum tiltangles of ±60°. The selector 291 may select orientation signal generatorP₁ 292 when the tilt detector 290 is at a level or null state (i.e., 0°tilt angle) relative to horizontal. When selected, orientation detectorP₁ 292 generates a unique orientation signal which is indicative of anorientation of 0°. As previously discussed, the orientation signal maybe combined with a signature signal produced by a separate signaturesignal generator 43 or, alternatively, may provide both signature signaland orientation signal information which is transmitted to the PDU 28.

In the event that the tilt detector 290 detects a positive 5° tilt anglechange, for example, orientation signal generator P₂ 293 is selected bythe selector 291. The orientation signal generator P₂ 293 then producesan orientation signal that indicates a positive 5° tilt condition.Similarly, orientation signal generators P₃ 294, P₄ 295, and P₅ 296 mayproduce orientation signals representing detected tilt angle changes ofpositive 10°, 15°, and 20°, respectively. Other orientation signalgenerators may be selected by the selector 291 to produce orientationsignals representing tilt angle changes in five degree incrementsbetween 25° and 60°. Negative tilt angles between 0° and -60° in 5°increments are preferably communicated to the PDU 28 by selection ofappropriate orientation signal generators corresponding to the magnitudeof negative tilting. It will be appreciated that the range andresolution between tilt angle increments may vary depending on aparticular application.

In FIGS. 11a and 11b, there is illustrated another embodiment of anunderground boring tool 500 equipped with a signature signal generatingapparatus which, in addition to providing location information, providesboring tool orientation information. Referring to FIG. 11a, the boringtool 500 includes a longitudinal axis 501 about which the boring tool500 rotates during boring activity. Distributed about the periphery ofthe boring tool 500 are a number of a signature signal generatingdevices, such as devices 504 and 508. In accordance with thisembodiment, the signature signal generating devices operate passivelyand, as such, do not require an external power supply. Each of thesignature signal generating devices distributed about the boring tool500 produces a unique signature signal in response to a received probesignal generated by the PDU 28.

As is further illustrated in FIG. 11b, the boring tool 500 includes anumber of elongated recesses or channels within which signature signalgenerating devices are disposed. In FIG. 11b, there is shown across-sectional view of the boring tool 500 illustrated in FIG. 11a. Asignature signal generating device 504, such as a co-axial transmissionline, for example, is disposed in a recess 502 and encased in aprotective material 505 which permits passage of electromagnetic signalstherethrough. The protective material 505 fixes the signature signalgenerating device 504 within the channel 502. Also shown in FIG. 11b isa second signature signal generating device 508 similarly disposed in arecess 506 and encased in a protective material 505. A hard dielectricmaterial, such as KEVLAR®, is a material suitable material for thisapplication.

During operation, the boring tool 500 is rotated at an appropriatedrilling rate which, assuming a full 360° rotation, exposes each of thesignature signal generating devices to a probe signal 36 produced by thePDU 28. When exposed to the probe signal 36 during rotation, each of thesignature signal generating devices will emit a characteristic orsignature signal 38 in response to the probe signal 36. As a particularsignature signal generating device rotates beyond a reception windowwithin which the probe signal 36 is received and a signature signal 38generated, the bulk metallic material of the boring tool 500 shieldssuch a signature signal generating device from the probe signal 36. Itmay be desirable to situate the signature signal generating devicesabout the periphery of the boring tool 500 such that the signaturesignal produced by the signature signal detecting device exposed to theprobe signal 36 produces the predominant signature signal 38 received bythe PDU 28. It may further be desirable to provide for a null or deadzone between adjacent signature signal generating devices so that theonly signature signal 38 received by the PDU 28 is that produced by asingle signature signal generating device currently exposed to the probesignal 38.

The type of signature signal generating device, configuration of theboring tool recesses, such as recess 502, the type of protectivematerial 505 employed, the number and location of signature signalgenerating devices used, and the rotation rate of the boring tool 500will typically impact the ability of the PDU 28 to detect the signaturesignal 38 produced by each of the signature signal generating devicesduring boring tool rotation.

Turning now to FIG. 12, there is illustrated an embodiment of anorientation detector suitable for use in both active and passivesignature signal generating apparatuses. In one embodiment, a mercurysensor 220 may be constructed having a bent tube 221 within which a beadof mercury 222 moves as the tube 221 tilts within a plane defined by theaxes 223 and 225. Pairs of electrical contacts, such as contacts 227 and229, are distributed along the base of the tube 221. As the tube 221tilts, the mercury bead 222 is displaced from an initial or null point,generally located at a minimum bend angle of the tube 221. As the bead222 moves along the tube base, electrical contact is mad betweenelectrical contact pairs 227 and 229 distributed along the tube base. Asthe amount of tube tilting increases, the mercury bead 222 is displacedfurther from the null point, thus completing electrical circuit pathsfor contact pairs located at corresponding further distances from thenull point. As such, the incremental change in tilt magnitude may bedetermined by detecting continuity in the contact pair over which themercury bead 222 is situated.

In one embodiment, sixty-four of such contact pairs are provided alongthe base of the tube 221 to provide 64-bit tilt resolution information.An electrical circuit or logic (not shown) is coupled to the pairs ofelectrical contacts 227 and 229 which provides an output indicative ofthe magnitude of tube tilting, and thus an indication of the magnitudeof the cooperative target orientation with respect to the plane definedby axes 223 and 225. It is appreciated that use of a mercury sensor 220in accordance with this embodiment may require a power source. As such,this embodiment of an orientation detector is appropriate for use inactive signature signal generating circuits. It is noted that the rangeof tilt angles detectable by the mercury sensor 220 is dependent on thebend angle α provided in the bent tube 221. The bend angle α, as well asthe length of the tube 221, will also impact the detection resolution ofmercury bead displacement within the tube 221.

In accordance with another embodiment of an orientation detectorsuitable for use in passive signature signal generating circuits,reference is made to FIGS. 12 and 13a-13b. The illustration of theapparatus depicted in FIG. 12 may be viewed in a context other than thatpreviously described in connection with a mercury sensor embodiment. Inparticular, a metallic ball or other metallic object 222 is displacedwithin a tube 221 in response to tilting of the tube 221 within theplane defined by the axes 223 and 225. The movable contact 222 movesalong a pair of contact rails 235a and 235b separated by a channel 237.The rails 235a and 235b include gaps 233 which separate one contact railcircuit from an adjacent contact rail circuit. As is illustrated indetail in FIGS. 13a-13b, each of the contact rail circuits is coupled toa pair of contacts 227 and 229 which, in turn, are coupled to atransmission line capable of producing a unique signature signal.

By way of example, and with particular reference to FIGS. 13a-13b,movable contact 222 is shown moving within the tube 221 between a firstposition P_(a) and a second position P_(b) in response to tilting of thetube 221. When the movable contact 222 is at the position P_(a),continuity is established between contract 227, contact rail 235a,movable contact 222, contact rail 235b, and contact 229. As such, thecircuit path including the transmission line T₄ 230 is closed. A probesignal 235 produced by the PDU 28 is received by the receive antenna 234which communicates the probe signal along an input waveguide 232 andthrough the circuit path defined by contact 227, rail contact 235a,movable contact 222, rail contact 235b, and contact 229. The receivedprobe signal 235 transmitted to the time-delaying waveguide T₄ 230produces a time-delayed signature signal which is communicated to anoutput waveguide 242 and to a transmit antenna 244. The signature signalproduced by the waveguide T₄ 230 is then received by the PDU 28. The PDU28 correlates the signature signal 245 with the selected signaturesignal waveguide, such as transmission line T₄ 230, and determines themagnitude of tube 221 tilting. Those skilled in the art will appreciatethat various impedance matching techniques, such as use of quarterwavelength matching stubs and the like, may be employed to improveimpedance matching within the waveguide pathways illustrated in FIGS.13a-13b.

Referring now to FIG. 14, there is illustrated another embodiment of anorientation detection apparatus suitable for detecting an orientation ofan underground boring tool 510. In accordance with this embodiment, anumber of rotation detectors, such as R₁ 512 and R₂ 514, are disposed atvarious radial locations about the periphery of the boring tool 510. Therotation detectors detect radial displacement of the boring tool 510 asthe boring tool 510 rotates about its longitudinal axis 501. A pitchdetector 516, oriented parallel with the longitudinal axis 501 of theboring tool 510, is susceptible to changes in boring tool pitch. In oneembodiment, the rotation detectors, such as R₁ 512 and R₂ 514, and thepitch detector 516 are accelerometer-type sensors. Alternatively, therotation and pitch detectors may constitute spring or strain gauge stylesensors. Various other known displacement sensor mechanisms may also beemployed.

The magnitude of the responsive of each rotation detector, such asdetector R₁ 512, is typically dependent on the radial location of aparticular rotation detector relative to earth's gravity vector as theboring tool 24 rotates about the longitudinal axis 501. The magnitude ofthe output produced by the pitch detector 516 is typically dependent onthe degree of a boring tool pitch off of horizontal relative to theground surface 11. The output signal produced by each of the rotationdetectors and the pitch detector may be encoded onto the signaturesignal produced by the signature signal generating apparatus provided onthe boring tool 510 or, alternatively, transmitted to the PDU 28 as aseparate information signal.

FIG. 21a illustrates yet another embodiment of an orientation sensingapparatus suitable for use with a boring tool 400. The boring tool 400incorporates a passive time domain signature signal circuit including asingle antenna 402, connected via a time delay line 404 to a termination406, as discussed hereinabove with respect to FIG. 5a. The circuitillustrated in FIG. 21a also includes a mercury switch 408 located at apoint along the delay line 404 close to the termination 406. Thetermination 406 also includes a dissipative load. When the boring tool400 is oriented so that the mercury switch 408 is open, the time domainsignature signal is generated by reflecting an incoming probe signal 407at the open circuit of the mercury switch 408. When the boring tool 400is oriented so that the mercury switch 408 is closed, the circuit fromthe antenna 402 is completed to the dissipative load 406 through thedelay line 404. The probe signal 407 does not reflect from thedissipative load 406 and therefore no signature signal is generated. Thegeneration of the signature signal 409 received by the PDU 28 is shownas a function of time in FIG. 21b. The I_(p) trace 407b shows the probesignal 407, I_(p), plotted as a function of time.

As the boring tool 400 rotates and moves along an underground path, theresistance, Rm, of the mercury switch 408 alternates from low to highvalues, as shown in the center trace 408b. The regular opening andclosing of the mercury switch 408 modulates the signature signal 409b,I_(s), received at the surface. The modulation maintains a constantphase relative to a preferred orientation of the boring tool 24. Thelower trace does not illustrate the delaying effects of the time delayline 404 since the time scales are so different (the time delay on thesignature signal 409 is of the order of 10 nanoseconds, while the timetaken for a single rotation of the boring tool 24 is typically between0.1 and 1 second). Detection of the modulated signature signal 409 bythe PDU 28 allows the operator to determine the orientation of theboring tool head. It is understood that the other embodiments ofsignature signal generation described hereinabove can also incorporate amercury switch 408 and, preferably, a dissipative load 406 in order togenerate a modulated signature signal 409 for purposes of detecting theorientation of the boring tool 24.

In FIG. 15, there is illustrated an apparatus for actively generating asignature signal and an orientation signal in an underground boring tool24. There is shown the head of a boring tool 24a. At the front end ofthe boring tool 24a is a cutter 120 for cutting through soil, sand,clay, and the like when forming an underground passage. A cut-awayportion of the boring tool wall 122 reveals a circuit board 124 which isdesigned to fit inside of the boring tool 24a. Attached to the circuitboard 124 is a battery 126 for providing electrical power. Alsoconnected to the circuit board 124 is an antenna 128 which is used toreceive an incoming probe signal 36 and transmit an outgoing signaturesignal 38. The antenna 128 may be located inside the boring tool 24a ormay be of a conformal design located on the surface of the boring tool24a and conforming to the surface contour. The boring tool 24a may alsocontain one or more sensors for sensing the environment of the boringtool 24a. Circuitry is provided in the boring tool 24a for relaying thisenvironmental information to the control unit 32 situated above-ground.The sensors, such as an orientation sensor 131, may be used to measure,for example, the orientation of the boring tool 24a, (pitch, yaw, androll) or other factors, such as the temperature of the cutting tool heador the pressure of water at the boring tool 24a.

In FIG. 15, there is illustrated a sensor 130, such as a pressuresensor, located behind the cutter 120. An electrical connection 132 runsfrom the sensor 130 to the circuit board 124 which contains circuitryfor analyzing the signal received from the sensor 130. The circuit board124 may modulate the signature signal 38 to contain information relatingto the sensor output or, alternatively, may generate separate sensorsignals which are subsequently detected and analyzed above-ground. Alsodepicted is an orientation sensor 131 which produces an orientationsignal indicative of an orientation of the boring tool 24, such as thelateral position or deviation of the boring tool 24 relative to apredefined underground path or, by way of further example, the pitch ofthe boring tool 24 relative to horizontal.

A methodology for detecting the depth of a boring tool 24 incorporatinga cooperative target 20 in accordance with one embodiment is illustratedin FIG. 16. In accordance with this embodiment, the PDU 28 includes atransmit antenna 250 and two receive antennas, AR₁ 252 and AR₂ 254. Eachof the receive antennas AR₁ 252 and AR₂ 254 is situated a known distance2m from the transmit antenna AT 250. It is assumed for purposes of thisexample that the propagation rate K through the ground medium ofinterest is locally constant. Although this assumption may introduce adegree of error with respect to actual or absolute depth boring tool,any such error is believed to be acceptable given the typicalapplication or use of the boring tool cooperative detection techniquedescribed here. In other applications, absolute depth determinations maybe desired. In such a case, the local propagation rate K, or dielectricconstant, may be empirically derived, one such procedure being describedhereinbelow.

Returning to FIG. 16, the time-of-flight, t₁, of the signal travelingbetween the cooperative target 20 of the boring tool 24 and the receiveantenna AR 252, and between the transmit antenna 250 and the cooperativetarget 20 of the boring tool 24 is determined when the cooperativetarget 20 is positioned below the centerline of the antennas AR₁ 252 andAT 250. The travel time of the signal traveling between the cooperativetarget 20 and the receive antenna AR₂ 254 is indicated as the time t₂.The depth d of the boring tool 24 that incorporates the cooperativetarget 20 may then be determined by application of the followingequations:

    d.sup.2 =K.sup.2 (t.sub.1.sup.2)-m.sup.2                   [ 1]

    d.sup.2 =K.sup.2 (t.sub.2.sup.2)-9m.sup.2                  [ 2]

    K.sup.2 (t.sub.2.sup.2)-K.sup.2 (t.sub.1.sup.2)=8m.sup.2   [ 3]

    K.sup.2 (t.sup.2.sup.2 -t.sub.1.sup.2)=8m.sup.2 [4]

    K.sup.2 =[8m.sup.2 /(t.sub.2.sup.2 -t.sub.1.sup.2)]        [5]

    d.sup.2 =[8m.sup.2 /(t.sub.2.sup.2 -t.sub.1.sup.2)](t.sub.1.sup.2)-m.sup.2[ 6]

    d=m[(8t.sub.1.sup.2 /(t.sub.2.sup.2 -t.sub.1.sup.2))-1].sup.2[ 7]

In accordance with an alternative approach for determining the depth dof a cooperative target 20, depth calculations may be based onfield-determined values for characteristic soil properties, such as thedielectric constant and wave velocity through a particular soil type. Asimplified empirical technique that may be used when calibrating thedepth measurement capabilities of a particular GPR system involvescoring a sample target, measuring its depth, and relating it to thenumber of nanoseconds it takes for a wave to propagate through the coresample.

For an embodiment of the invention which uses a microwave probe signal,a general relationship for calculating the depth or dielectric constantfrom the time of flight measurement is described by the followingequation: ##EQU1## where, TE is an effective time-of-flight, which isthe duration of time during which a probe signal or signature signal istraveling through the ground; TF is the measured time-of-flight; TD isthe delay internal to the cooperative target between receiving the probesignal and transmitting the signature signal; d_(j) is the thickness ofthe jth ground type above the cooperative target; ε_(j) is the averagedielectric constant of the jth ground type at the microwave frequency;and c is the speed of light in a vacuum. It is important to know thedielectric constant since it provides information related to the type ofsoil being characterized and its water content. Having determined thedielectric constant of a particular soil type, the depth of the boringtool 24 traversing through similar soil types can be directly derived byapplication of the above-described equations.

A methodology for detecting the location of an underground boring tool24 as the boring tool 24 creates or otherwise travels along anunderground path is illustrated in FIGS. 17a-17b and 18a-18b. Withreference to these figures and to FIG. 1, an underground boringoperation is depicted in which a boring tool 24 is shown excavating theground 10 so as to create an underground path or borehole 26. The drillstring 22 is increased in length during the boring operation typicallyby adding individual drill string members 23 to the drill string 22 in amanner previously discussed. As the drill string length is increased,and the boring tool 24 forced further into the ground 10, the PDU 28 ismoved along a preferred above-ground path 41 at a speed approximatelyequal to the horizontal speed component of the boring tool 24.

In one embodiment, the PDU 28 repeatedly transmits a probe signal 36into the ground 10 when moved along the path 41, which is received bythe signature signal generating apparatus provided on or within theboring tool 24. In response to the probe signal 36, a signature signal38 is transmitted at the boring tool 24 and received by the PDU 28. Anydeviation taken by the boring tool 24 from the preferred above-groundpath 41 is detected by the PDU 28. An appropriate course correction maybe effected either manually or automatically by the trenchlessunderground boring system 12 in response to such a deviation, as will bediscussed hereinbelow. While effecting a boring tool course change, thePDU 28 is moved along the path 41 so as to continue tracking theprogress and direction of the boring tool 24 through the ground 10. Inthis manner, cooperation between the PDU 28, the boring tool 24, and theabove-ground portion of the trenchless underground boring system 12provide for reliable and accurate navigating and tracking of anunderground boring tool 24 during excavation.

FIGS. 17a and 17b illustrate one embodiment of a detection methodologyemploying an antenna array 37 coupled to the PDU 28. The antenna array37 includes a left receive antenna A_(L) and a right receive antennaA_(R) which are respectively positioned on either side of a transmitantenna (not shown) situated at a mid-point between the two receiveantennas A_(L) and A_(R). The dashed line 41 shown in FIG. 17a depicts apreferred above-ground path under which a borehole 26 is to be created,or has been created, by a boring tool 24 equipped with a cooperativetarget 20. At a first location L1, it can be seen that the undergroundboring tool 24 is located immediately beneath the transmit antennapositioned in the center of the antenna array 37. A probe signal 36emitted by the transmit antenna at a time t₀ is received by thecooperative target 20 in the boring tool 24, which, in turn, produces asignature signal 38 that is received by the left receive antenna A_(L)and the right receive antenna A_(R) at approximately the same time, asis illustrated in the graph G₁ of FIG. 17b.

Referring to the graph G₁ of FIG. 17b, it is assumed that the probesignal 36 produced by the PDU 28 is transmitted at a time t₀. Becausethe two receive antennas A_(L) and A_(R) of the antenna array 37 aresubstantially equidistant relative to the cooperative target 20, thesignature signal produced by the cooperative target 20 is received bythe two antennas A_(L) and A_(R) at substantially the same time, t₁,after transmission of the probe signal at time t₀. Concurrent receptionof the signature signal by the two receive antennas A_(L) and A_(R) isdepicted in the graph G₁ of FIG. 17b as detected signals S_(R) andS_(L), respectively, at a time t₁.

At a second location L2 along the preferred or predeterminedabove-ground path 41, it can be seen that the boring tool 24 hasdeviated in a direction left (L) of the center (C) of the predeterminedpath 41. This deviation of the boring tool 24 is detected by the PDU 28as a time delay between a time the signature signal 38 is received bythe left and right receive antennas A_(L) and A_(R), respectively. Thistime delay results from a difference in the separation distance betweenthe boring tool 24 with respect to the left and right receive antennasA_(L) and A_(R). It can be seen that the separation distance between theleft receive antenna A_(L) and the boring tool 24 is less than theseparation distance between the right receive antenna A_(R) and theboring tool 24. The boring tool deviation from the center of path 41 isreflected in the graph G₂ of FIG. 17b as a delay between reception ofthe signature signal S_(L) by the left receive antenna A_(L) at a timet₂ and reception of the signature signal S_(R) by the right receiveantenna A_(R) at a later time t₃.

At a third location L3 further along the preferred path 41, it can beseen that the boring tool 24 has deviated to the right (R) of the center(C) of the preferred path 41. Such a deviation may result fromovercompensation when effecting a course change from a left-of-centerlocation, such as from the second location L2. The right-of-center driftof the boring tool 24 is detected by the PDU 28 as the relative timedelay between signature signal reception by the left and right receiveantennas A_(L) and A_(R), respectively. At the location L3, it can beseen that the distance between the boring tool 24 and the right receiveantenna A_(R) is less than the distance between the boring tool 24 andthe left receive antenna A_(L). Accordingly, as is indicated in thegraph G₃ of FIG. 17b, the signature signal S_(R) is received by theright receive antenna A_(R) in advance of the signature signal S_(L)received by the left receive antenna A_(L), thereby resulting in a timedelay defined as Δ(t₃ -t₂). This relative time delay may be used todetermine the magnitude of boring tool deviation from the predeterminedpath 41.

At a fourth location L4 along the predefined-above-ground path 41, itcan be seen that the boring tool 24 has been directed to the desiredcenter point location along the path 41 after having deviated to theright of the path center point at the previously discussed location L3.As is shown at location L4, the boring tool 24 is again orientatedimmediately below the center point of the antenna array 37. Thesignature signal 38 produced by the cooperative target 20 in response toa probe signal 36 emitted from the transmit antenna situated within theantenna array 37 is received substantially concurrently by the left andright receive antennas A_(L) and A_(R). The graph G₄ of FIG. 17bdemonstrates that the boring tool 24 is once again progressing asdesired along the center line of the predetermined path 41, as evidencedby contemporaneous reception of the signature signal 38 by the left andright receive antennas A_(L) and A_(R), respectively. It is noted thatthe depth of the boring tool, d, may be determined by any of theapproaches discussed herein above. In addition, orientation of theboring tool 20 may also be detected and determined in a mannerpreviously discussed above.

FIGS. 18a-18b illustrate another embodiment of an antenna arrayconfiguration which may be employed in combination with the PDU 28 toaccurately track the progress of the underground boring tool 24 along anunderground path. With reference to FIG. 18a, an antenna array 37includes four receive antennas A₁, A₂, A₃, and A₄. The antenna array 37also includes a transmit antenna (not shown) situated at a locationwithin the array 37, typically at a center location. In accordance withthis embodiment, the four receive antennas are distributed about thecircular array 37 at 0°, 90°, 180°, and 270° positions, respectively. Itis to be understood that the configuration of the antenna array 37 neednot be circular as is illustrated in the figures, but may instead bearranged in any suitable geometric configuration. Also, the distributionof receive antennas about the antenna array may be different from thatillustrated in the figures.

FIG. 18a is a depiction of the antenna array 37 having its centertransmit antenna orientated co-parallel with a predeterminedabove-ground path 41. Superimposed in FIG. 18a is an underground boringtool 24 equipped with a cooperative target 20 depicted at threedifferent locations L1, L2, and L3 along the predetermined path 41. Atthe location L1, it can be seen that the boring tool 24 is properlyaligned co-parallel with the preferred path 41. The signature signalproduced by the cooperative target 20, in response to a probe signalproduced by the transmit antenna at a time t₀, is received atsubstantially the same time, t₄, by each of the four receive antennasA₁, A₂, A₃, and A₄. The in-phase relationship of the signature signalsS₁, S₂, S₃, and S₄ respectively received by receive antennas A₁, A₂, A₃and A₄ is depicted in the graph G₁ of FIG. 18b.

At a location L2, it can be seen that the boring tool 24 has deviatedright-of-center with respect to the path 41. This course deviation takenby the boring tool 24 is detected by the PDU 28 as an out-of-phasesignature signal response within the antenna array 37. Theright-of-center deviation is demonstrated in the graph G₂ of FIG. 18b bythe signature signal reception relationship associated with each of thefour receive antennas A₁, A₂, A₃, and A₄. It can be seen that thedistance between the boring tool 24 at location L2 and the receiveantenna A₂ is less than the distance between the boring tool 24 and theother receive antennas A₁, A₃, and A₄. As is depicted in the graph G₂ ofFIG. 18b, the signature signal S₂ is received at a time t₂ by thereceive antenna A₂ earlier than the reception times associated with theother receive antennas. By way of further example, the relativedistances between the cooperative target 24 and the receive antennas A₁and A₄ at the previous location L1 have effectively increased when theboring tool 24 deviates to the location L2, thereby increasing the delaytime of signature signal reception by receive antennas A₁ and A₄. Assuch, reception of the signature signal S₁ by antenna A₁ at a time t₇and the signature signal S₄ by receive antenna A₄ at a time t₈ isdelayed with respect to the reception of signature signal received byreceive antennas A₂ and A₃ at times t₂ and t₅ respectively.

At a location L3, the graph G₃ of FIG. 18b demonstrates that the boringtool 24 has deviated to a left-of-center position relative to the path41. The magnitude of the relative time delay within the antenna array 37indicates the magnitude of off-of-center boring tool deviations as isillustrated by the signature signal response graph of FIG. 18b. It isnoted that the boring tool 24 may deviate beyond the periphery of theantenna array 37. Such a deviation will result in a more pronouncedreduction in the signal-to-noise ratio with respect to receive antennassituated furthest away from the boring tool location. It is understoodthat an increase in the number of receive antennas within the antennaarray 37 provides for a concomitant increase in boring tool detectionresolution. It is believed that an antenna array 37 having a diameterranging between approximately 2 feet and 5 feet is sufficient fordetecting the location of the boring tool 24 at depths of approximately10 to 15 feet or less.

In order to obtain three-dimensional data, a GPR system employingsingle-axis antenna must make several traverses over the section ofground or must use multiple antennae. The following describes theformation of two and three dimensional images in accordance with anotherembodiment of an antenna configuration used in combination with the PDU28. In FIG. 19, there is shown a section of ground 500 for which a PDU28, typically including a GPR forms an image, with a buried hazard 502located in the section of ground 500. The ground surface 504 lies in thex-y plane formed by axes x and y, while the z-axis is directedvertically into the ground 500. Generally, a single-axis antenna, suchas the one illustrated as antenna-A 506 and oriented along the z-axis,is employed to perform multiple survey passes 508. The multiple surveypasses 508 are straight line passes running parallel to each other andhave uniform spacing in the y direction. The multiple passes shown inFIG. 19 run parallel to the x-axis.

Generally, as discussed previously, a GPR system has a time measurementcapability which allows measuring of the time for a signal to travelfrom the transmitter, reflect off of a target, and return to thereceiver. After the time function capability of the GPR system providesthe operator with depth information, the radar system is moved laterallyin a horizontal direction parallel to the x-axis, thus allowing for theconstruction of a two-dimensional profile of a subsurface. By performingmultiple survey passes 508 in a parallel pattern over a particular site,a series of two-dimensional images can be accumulated to produce anestimated three-dimensional view of the site within which a buriedhazard may be located. It can be appreciated, however, that thetwo-dimensional imaging capability of a conventional antennaconfiguration 506 may result in missing a buried hazard, particularlywhen the hazard 502 is parallel to the direction of the multiple surveypasses 508 and lies in between adjacent survey passes 508.

A significant advantage of a geologic imaging antenna configuration 520of the present invention provides for true three-dimensional imaging ofa subsurface as shown in FIG. 20. A pair of antennae, antenna-A 522 andantenna-B 524, are preferably employed in an orthogonal configuration toprovide for three-dimensional imaging of a buried hazard 526. Antenna-A522 is shown as directed along a direction contained within the y-z axisand at +45° relative to the z-axis. Antenna-B 524 is also directed alonga direction contained within the y-z plane, but at -45° relative to thez-axis, in a position rotated 90° from that of antenna-A 522. It isnoted that the hyperbolic time-position data distribution typicallyobtained by use of a conventional single-axis antenna, may instead beplotted as a three-dimensional hyperbolic shape that provides width,depth, and length dimensions of a detected buried hazard 526. It isfurther noted that a buried hazard 526, such as a drainage pipeline,which runs parallel to the survey path 528 will readily be detected bythe three-dimensional imaging GPR system. Respective pairs oforthogonally oriented transmitting and receive antennae may be employedin the transmitter 54 and receiver 56 of the PDU 28 in accordance withone embodiment of the invention.

Additional features can be included on the boring tool 24. It may bedesired, under certain circumstances, to make certain measurements ofthe boring tool 24 orientation, shear stresses on the drill string 22,and the temperature of the boring tool 24, for example, in order to moreclearly understand the conditions of the boring operation. Additionally,measurement of the water pressure at the boring tool 24 may provide anindirect measurement of the depth of the boring tool 24 as previouslydescribed hereinabove.

FIG. 21c illustrates an embodiment which allows sensors to sense theenvironment of the boring tool 410. The figure shows an active timedomain signature signal generation circuit which includes a receiveantenna 412 connected to a transmit antenna 414 through an active timedomain circuit 416. A sensor 418 is connected to the active time domaincircuit 416 via a sensor lead 420. In this embodiment, the sensor 418 isplaced at the tip of the boring tool 410 for measuring the pressure ofwater at the boring tool 410. The reading from the sensor 418 isdetected by the active time domain circuit 416 which converts thereading into a modulation signal. The modulation signal is subsequentlyused to modulate the actively generated signature signal 415. Thisprocess is described with reference to FIG. 21d, which shows severalsignals as a function of time. The top signal 413d represents the probesignal, I_(p), received by the receive antenna 412. The second signal,415d, represents the actively generated signature signal I_(a), whichwould be generated if there were no modulation of the signature signal.The third trace, 416d, shows the amplitude modulation signal, I_(m),generated by the active time domain circuit 416, and the last trace,422d, shows the signature signal, I_(s), after amplitude modulation. Themodulated signature signal 415 is detected by the PDU 28. Subsequentdetermination of the modulation signal by the signal processor 60 in thePDU 28 provides data regarding the output from the sensor 418.

Modulation of the signature signal is not restricted to the combinationof amplitude modulation of a time domain signal as shown in theembodiment of FIG. 21. This combination was supplied for Illustrativepurposes only. It is understood that other embodiments include amplitudemodulation of frequency domain signature signals, and frequencymodulation of both time and frequency domain signature signals. Inaddition, the boring tool 24 may include two or more sensors rather thanthe single sensor as illustrated in the above embodiment.

FIG. 22a illustrates another embodiment of the invention in which aseparate active beacon is employed for transmitting information on theorientation or the environment of the boring tool 430 to the PDU 28. Inthis embodiment, shown in FIG. 22a, the boring tool 430 includes apassive time domain signature circuit employing a single antenna 432, atime delay line 434, and an open termination 436 for reflecting theelectrical signal. The single antenna 432 is used to receive a probesignal 433 and transmit a signature/beacon signal 435. An active beaconcircuit 438 generates a beacon signal, preferably having a selectedfrequency in the range of 50 KHz to 500 MHz, which is mixed with thesignature signal generated by the termination 436 and transmitted fromthe antenna 432 as the composite signature/beacon signal 435. A mercuryswitch 440 is positioned between the active beacon circuit 438 and theantenna 432 so that the mercury switch 440 operates only on the signalfrom the active beacon circuit 438 and not on the signature signalgenerated by the termination 436.

When the boring tool 430 is oriented so that the mercury switch 440 isopen, the beacon signal circuit 438 is disconnected from the antenna432, and no signal is transmitted from the active beacon circuit 438.When the boring tool 430 is oriented so that the mercury switch 440 isclosed, the active beacon circuit 438 is connected to the antenna 432and the signal from the active beacon circuit 438 is transmitted alongwith the signature signal as the signature/beacon signal 435. The effectof the mercury switch on the signature/beacon signal 435 has beendescribed previously with respect to FIG. 21b. The top trace 438b, inFIG. 22b, shows the signal, I_(b), generated by the active beaconcircuit 438 as a function of time. As the boring tool 430 rotates andmoves along an underground path, the resistance, Rm, of the mercuryswitch 440 alternates from low to high values, as shown in the centertrace 440b. The continual opening and closing of the mercury switch 440produces a modulated signature/beacon signal 435b, I_(m), which isreceived at the surface by the PDU 28. Only a beacon signal component,and no signature signal component, is shown in signal I_(m) 435b. Themodulation of signal I_(m) 435b maintains a constant phase relative to apreferred orientation of the boring tool 430. Analysis of the modulationof the beacon signal by a beacon receiver/analyzer 61 on the PDU 28allows the operator to determine the orientation of the boring toolhead.

FIG. 22c illustrates an embodiment which allows sensors to sense theenvironment of the boring tool 450 where an active beacon is used totransmit sensor data. The figure shows an active time domain signaturesignal generation circuit including a receive antenna 452, a transmitantenna 454, and an active time domain signature signal circuit 456, allof which are connected via a time delay line 457. An active beaconcircuit 460 is also connected to the transmit antenna 454. A sensor 458is connected to the active beacon circuit 460 via a sensor lead 462. Inthis embodiment, the sensor 458 is placed near the tip of the boringtool 450 and is used to measure the pressure of water at the boring tool450. The sensor reading is detected by the active beacon circuit 460which converts the signal from the sensor 458 into a modulation signal.The modulation signal is subsequently used to modulate an active beaconsignal generated by the active beacon circuit 460.

To illustrate the generation of the signature/beacon signal 455transmitted to the PDU 28, several signals are illustrated as a functionof time in FIG. 22d. The signal 453d represents the probe signal, I_(p),received by the receive antenna 452. The second signal 456d representsthe time-delayed signature signal, I_(s), generated by the active timedomain circuit 456. The third signal 460d, I_(c), represents acombination of the time-delayed signature signal I_(s) 456d and anunmodulated signal produced by the active beacon circuit 460. The lasttrace, 455d, shows a signal received at the surface, I_(m), which is acombination of the time-delayed signature signal I_(s) 456d and a signalproduced by the active beacon circuit 460 which has been modulated inaccordance with the reading from the sensor 458. Detection of themodulated active beacon signal by the beacon signal detector 61 in thePDU 28, followed by appropriate analysis, provides data to the userregarding the output from the sensor 458.

In FIG. 23, there is illustrated an embodiment for using a detectionsystem to locate an underground boring tool and to characterize theintervening medium between the boring head and the PDU 28. In thisfigure, there is illustrated a trenchless underground boring system 12situated on the surface 11 of the ground 10 in an area in which theboring operation is to take place. A control unit 32 is located near thetrenchless underground boring system 12. In accordance with thisillustrative example, a boring operation is taking place under aroadway. The ground 10 is made up of several different ground types, theexamples as shown in FIG. 23 being sand (ground type (GT2)) 140, clay(GT3) 142 and native soil (GT4) 144. The road is generally described bythe portion denoted as road fill (GT1) 146. FIG. 12 illustrates a drillstring 22 in a first position 22c, at the end of which is located aboring tool 24c. The PDU 28c is shown as being situated at a locationabove the boring tool 24c. The PDU 28c transmits a probe signal 36cwhich propagates through the road fill and the ground.

In the case of the boring tool at location 24c, the probe signal 36cpropagates through the road fill 146 and the clay 142. The boring tool24c, in response, produces a signature signal 38c which is detected andanalyzed by the PDU 28c. The analysis of the signature signal 38cprovides a measure of the time-of-flight of the probe signal 36c and thesignature signal 38c. The time-of-flight is defined as a time durationmeasured by the PDU 28c between sending the probe signal 36c andreceiving the signature signal 38c. The time-of-flight measured dependson a number of factors including the depth of the boring tool 24c, thedielectric conditions of the intervening ground medium 146 and 142, andany delay involved in the generation of the signature signal 38c.Knowledge of any two of these factors will yield the third from thetime-of-flight measurement.

The depth of the boring tool 24c can be measured independently using amechanical probe or sensing the pressure of the water at the boring tool24c using a sensor 130 located in the boring tool head 24c as discussedhereinabove. For the latter measurement, the boring operation is halted,and the water pressure measured. Since the height of the water column inthe drill string 22 above the ground is known, the depth of the boringtool 24c can be calculated using known techniques. For an embodiment ofthe invention which uses a microwave probe signal, a generalrelationship for calculating the depth or dielectric constant from thetime of flight measurement is given by Equation [8] discussed previouslyhereinabove.

For the case where the boring tool is located at position 24c as shownin FIG. 23, and with the assumption that the road fill has a negligiblethickness relative to the thickness of clay, the relationship ofEquation [8] simplifies to: ##EQU2## where, the subscript "3" refers toGT3. Direct measurement of the time-of-flight, TF, and the depth of theboring tool 24c, d₃, along with the knowledge of any time delay, TD,will yield the average dielectric constant, ε₃, of GT3. Thischaracteristic can be denoted as GC3.

Returning to FIG. 23, there is illustrated an embodiment in which theboring tool 24 has been moved from its first location 24c to anotherposition 24d. The drill string 22d (shown in dashed lines) has beenextended from its previous configuration 22c by the addition of extradrill string members in a manner as described previously hereinabove.The PDU 28 has been relocated from its previous position 28c to a newposition 28d (shown in dashed lines) in order to be close to the boringtool 24d. The parameter GC4, which represents the ground characteristicof the native soil GT4, can be obtained by performing time-of-flightmeasurements as previously described using the probe signal 36d andsignature signal 38d. Likewise, ground characteristic GC2 can beobtained from time-of-flight measurements made at the point indicated bythe letter "e". The continuous derivation of the ground characteristicsas the boring tool 24d travels through the ground results in theproduction of a ground characteristic profile which may be recorded bythe control unit 32. The characteristics of the intervening groundmedium between the PDU 28 and the cooperative target 20 may bedetermined in manner described herein and in U.S. Pat. No. 5,553,407,which is assigned to the assignee of the instant application, thecontents of which are incorporated herein by reference.

It may be advantageous to make a precise recording of the undergroundpath traveled by the boring tool 24. For example, it may be desirable tomake a precise record of where utilities have been buried in order toproperly plan future excavations or utility burial and to avoidunintentional disruption of such utilities. Borehole mapping can beperformed manually by relating the boring tool position data collectedby the PDU 28 to a base reference point, or may be performedelectronically using a Geographic Recording System (GRS) 150 showngenerally as a component of the control unit 32 in FIG. 24. In oneembodiment, a Geographic Recording System (GRS) 150 communicates with acentral processor 152 of the control unit 32, relaying the preciselocation of the PDU 28. Since the control unit 32 also receivesinformation regarding the position of the boring tool 24 relative to thePDU 28, the precise location of the boring tool 24 can be calculated andstored in a route recording database 154.

In accordance with another embodiment, the geographic position dataassociated with a predetermined underground boring route is acquiredprior to the boring operation. The predetermined route is calculatedfrom a survey performed prior to the boring operation. The prior surveyincludes GPR sensing and geophysical data in order to estimate the typeof ground through which the boring operation will take place, and todetermine whether any other utilities or buried hazards are located on aproposed boring pathway. The result of the pre-bore survey is apredetermined route data set which is stored in a planned route database156. The predetermined route data set is uploaded from the planned routedatabase 156 into the control unit 32 during the boring operation toprovide autopilot-like directional control of the boring tool 24 as itcuts its underground path. In yet another embodiment, the position dataacquired by the GRS 150 is preferably communicated to a route mappingdatabase 158 which adds the boring pathway data to an existing databasewhile the boring operation takes place. The route mapping database 158covers a given boring site, such as a grid of city streets or a golfcourse under which various utility, communication, plumbing and otherconduits may be buried. The data stored in the route mapping database158 may be subsequently used to produce a survey map that accuratelyspecifies the location and depth of various utility conduits buried in aspecific site. The data stored in the route mapping database 158 alsoincludes information on boring conditions, ground characteristics, andprior boring operation productivity, so that reference may be made bythe operator to all prior boring operational data associated with aspecific site.

An important feature of the novel system for locating the boring tool 24concerns the acquisition and use of geophysical data along the boringpath. A logically separate Geophysical Data Acquisition Unit 160 (GDAU),which may or may not be physically separate from the PDU 28, may providefor independent geophysical surveying and analysis. The GDAU 160preferably includes a number of geophysical instruments which provide aphysical characterization of the geology for a particular boring site. Aseismic mapping module 162 includes an electronic device consisting ofmultiple geophysical pressure sensors. A network of these sensors isarranged in a specific orientation with respect to the trenchlessunderground boring system 12, with each sensor being situated so as tomake direct contact with the ground. The network of sensors measuresground pressure waves produced by the boring tool 24 or some otheracoustic source. Analysis of ground pressure waves received by thenetwork of sensors provides a basis for determining the physicalcharacteristics of the subsurface at the boring site and also forlocating the boring tool 24. These data are processed by the GDAU 160prior to sending analyzed data to the central processor 152.

A point load tester 164 may be employed to determine the geophysicalcharacteristics of the subsurface at the boring site. The point loadtester 164 employs a plurality of conical bits for the loading pointswhich, in turn, are brought into contact with the ground to test thedegree to which a particular subsurface can resist a calibrated level ofloading. The data acquired by the point load tester 164 provideinformation corresponding to the geophysical mechanics of the soil undertest. These data may also be transmitted to the GDAU 160.

The GDAU 160 may also include a Schmidt hammer 166 which is ageophysical instrument that measures the rebound hardnesscharacteristics of a sampled subsurface geology. Other geophysicalinstruments may also be employed to measure the relative energyabsorption characteristics of a rock mass, abrasivity, rock volume, rockquality, and other physical characteristics that together provideinformation regarding the relative difficulty associated with boringthrough a given geology. The data acquired by the Schmidt hammer 166 arealso stored in the GDAU 160.

In the embodiment illustrated in FIG. 24, a Global Positioning System(GPS) 170 is employed to provide position data for the GRS 150. Inaccordance with a U.S. Government project to deploy twenty-fourcommunication satellites in three sets of orbits, termed the GlobalPositioning System (GPS), various signals transmitted from one or moreGPS satellites may be used indirectly for purposes of determiningpositional displacement of a boring tool 24 relative to one or moreknown reference locations. It is generally understood that the U.S.Government GPS satellite system provides for a reserved, or protected,band and a civilian band. Generally, the protected band provides forhigh-precision positioning to a classified accuracy. The protected band,however, is generally reserved exclusively for military and othergovernment purposes, and is modulated in such a manner as to render itvirtually useless for civilian applications. The civilian band ismodulated so as to significantly reduce the accuracy available,typically to the range of one hundred to three hundred feet.

The civilian GPS band, however, can be used indirectly in relativelyhigh-accuracy applications by using one or more GPS signals incombination with one or more ground-based reference signal sources. Byemploying various known signal processing techniques, generally referredto as differential global positioning system (DGPS) signal processingtechniques, positional accuracies on the order of centimeters are nowachievable. As shown in FIG. 24, the GRS 150 uses the signal produced byat least one GPS satellite 172 in cooperation with signals produced byat least two base transponders 174, although the use of one basetransponder 174 may be satisfactory in some applications. Various knownmethods for exploiting DGPS signals using one or more base transponders174 together with a GPS satellite 172 signal and a mobile GPS receiver176 coupled to the control unit 32 may be employed to accurately resolvethe boring tool 24 movement relative to the base transponder 174reference locations using a GPS satellite signal source.

In another embodiment, a ground-based positioning system may be employedusing a range radar system 180. The range radar system 180 includes aplurality of base radio frequency (RF) transponders 182 and a mobiletransponder 184 mounted on the PDU 28. The base transponders 182 emit RFsignals which are received by the mobile transponder 184. The mobiletransponder 184 includes a computer which calculates the range of themobile transponder 184 relative to each of the base transponders 182through various known radar techniques, and then calculates its positionrelative to all base transponders 182. The position data set gathered bythe range radar system 180 is transmitted to the GRS 150 for storing inroute recording database 154 or the route mapping system 158.

In yet another embodiment, an ultrasonic positioning system 190 may beemployed together with base transponders 192 and a mobile transponder194 coupled to the PDU 28. The base transponder 192 emits signals havinga known clock timebase which are received by the mobile transponder 194.The mobile transponder 194 includes a computer which calculates therange of the mobile transponder 194 relative to each of the basetransponders 192 by referencing the clock speed of the source ultrasonicwaves. The computer of the mobile transponder 194 also calculates theposition of the mobile transponder 194 relative to all of the basetransponders 192. It is to be understood that various other knownground-based and satellite-based positioning systems and techniques maybe employed to accurately determine the path of the boring tool 24 alongan underground path.

FIG. 25 illustrates an underground boring tool 24 performing a boringoperation along an underground path at a boring site. An importantadvantage of the novel geographic positioning unit 150, generallyillustrated in FIG. 25, concerns the ability to accurately navigate theboring tool 24 along a predetermined boring route and to accurately mapthe underground boring path in a route mapping database 158 coupled tothe control unit 32. It may be desirable to perform an initial survey ofthe proposed boring site prior to commencement of the boring operationfor the purpose of accurately determining a boring route which avoidsdifficulties, such as previously buried utilities or other obstacles,including rocks, as is discussed hereinbelow.

As the boring tool 24 progresses along the predetermined boring route,actual positioning data are collected by the geographic recording system150 and stored in the route mapping database 158. Any intentionaldeviation from the predetermined route stored in the planned pathdatabase 156 is accurately recorded in the route mapping database 158.Unintentional deviations are corrected so as to maintain the boring tool24 along the predetermined underground path. Upon completion of a boringoperation, the data stored in the route mapping database 158 may bedownloaded to a personal computer (not shown) to construct an "as is"underground map of the boring site. Accordingly, an accurate map ofutility or other conduits installed along the boring route may beconstructed from the route mapping data and subsequently referenced bythose desiring to gain access to, or avoid, such buried conduits.

Still referring to FIG. 25, accurate mapping of the boring site may beaccomplished using a global positioning system 170, range radar system180 or ultrasonic positioning system 190 as discussed previously withrespect to FIG. 24. A mapping system having a GPS system 170 includesfirst and second base transponders 600 and 602 together with one or moreGPS signals 606 and 608 received from GPS satellites 172. A mobiletransponder 610, coupled to the control unit 32, is provided forreceiving the GPS satellite signal 606 and base transponder signals 612and 614 respectively transmitted from the transponders 600 and 602 inorder to locate the position of the control unit 32. As previouslydiscussed, a modified form of differential GPS positioning techniquesmay be employed to enhance positioning accuracy to the centimeter range.A second mobile transponder 616, coupled to the PDU 28, is provided forreceiving the GPS satellite signal 608 and base transponder signals 618and 620 respectively transmitted from the transponders 600 and 602 inorder to locate the position of the PDU 28.

In another embodiment, a ground-based range radar system 180 includesthree base transponders 600, 602, and 604 and mobile transponders 610and 616 coupled to the control unit 32 and PDU 28, respectively. It isnoted that a third ground-based transponder 604 may be provided as abackup transponder for a system employing GPS satellite signals 606 and608 in cases where GPS satellite signal 606 and 608 transmission istemporarily terminated, either purposefully or unintentionally. Positiondata for the control unit 32 are processed and stored by the GRS 150using the three reference signals 612, 614, and 622 received from theground-based transponders 600, 602, and 604, respectively. Position datafor the PDU 28, obtained using the three reference signals 618, 620, and624 received respectively from the ground-based transponders 600, 602,and 604, are processed and stored by the local position locator 616coupled to the PDU 28 and then sent to the control unit 32 via a datatransmission link 34. An embodiment employing an ultrasonic positioningsystem 190 would similarly employ three base transponders 600, 602, and604, together with mobile transponders 610 and 616 coupled to thecontrol unit 32 and PDU 28, respectively.

Referring now to FIG. 26, there is illustrated in flowchart formgeneralized steps associated with the pre-bore survey process forobtaining a pre-bore site map and determining the optimum route for theboring operation prior to commencing the boring operation. In brief, apre-bore survey permits examination of the ground through which theboring operation will take place and a determination of an optimumroute, an estimate of the productivity, and an estimate of the cost ofthe entire boring operation.

Initially, as shown in FIG. 26, a number of ground-based transpondersare positioned at appropriate locations around the boring site at step300. The control unit 32 and the PDU 28 are then situated at locationsL0 and L1 respectively at step 302. The geographical recording system150 is then initialized and calibrated at step 304 in order to locatethe initial positions of the control unit 32 and PDU 28. Aftersuccessful initialization and calibration, the PDU 28 is moved along aproposed boring route, during which PDU data and geographical locationdata are acquired at steps 306 and 308, respectively. The data gatheredby the PDU 28 are preferably analyzed at steps 306 and 308. Theacquisition of data continues at step 312 until the expected end of theproposed boring route is reached, at which point data accumulation ishalted, as indicated at step 314.

The acquired data are then downloaded to the control unit 32, which maybe a personal computer, at step 316. The control unit 32, at step 318,then calculates an optimum predetermined path for the boring operation,and does so as to avoid obstacles and other structures. If it is judgedthat the predetermined route is satisfactory, as is tested at step 320,the route is then loaded into the planned path database 156 at step 322,and the pre-bore survey process is halted at step 324. If, however, itis determined that the planned route is unsatisfactory, as is tested atstep 320 because, for example, the survey revealed that the boring tool24 would hit a rock obstacle or that there were buried utilities whichcould be damaged during a subsequent boring operation, then the PDU 28can be repositioned, at step 326, at the beginning of the survey routeand a new route surveyed by repeating steps 304-318. After asatisfactory route has been established, the pre-bore survey process ishalted at step 324.

In another embodiment, the pre-bore survey process includes thecollection of geological data along the survey path, concurrently withposition location and PDU data collection. This collection activity isillustrated in FIG. 26 which shows an initialization and calibrationstep 328 for the geophysical data acquisition unit 160 (GDAU) takingplace concurrently with the initialization and calibration of thegeographical recording system 150. The GDAU 160 gathers geological dataat step 330 at the same time as the PDU 28 and position data are beingacquired in steps 306 and 308, respectively. The inclusion of geologicaldata gathering provides for a more complete characterization of theground medium in the proposed boring pathway, thus allowing for moreaccurate productivity and cost estimates to be made for the boringoperation.

In a third embodiment, the survey data are compared with previouslyacquired data stored in the route mapping database 158 in order toprovide estimates of the boring operation productivity and cost. In thisembodiment, historical data from the route mapping database are loadedinto the control processor 152 at step 332 after the survey data havebeen downloaded to the control unit 32 in step 316. The data downloadedfrom the route mapping database 158 include records of prior surveys andboring operations, such as GPR and geological characterizationmeasurements and associated productivity data. The pre-planned route iscalculated at step 334 in a manner similar to the calculation of theroute indicated at step 318. By correlating the current groundcharacterization, resulting from the PDU 28 and GDAU 160 data, withprior characterization measurements and making reference to associatedprior productivity results, it is possible to estimate, at step 336,productivity data for the planned boring operation. Using the estimatedproduction data of step 336, it is then possible to produce a costestimate of the boring process at step 338. In the following step 320, adetermination is made regarding whether or not the pre-planned route issatisfactory. This determination can be made using not only thesubsurface features as in the first embodiment, but can be made usingother criteria, such as the estimated duration of the boring process orthe estimated cost.

Referring now to FIG. 27, there is illustrated a system block diagram ofa control unit 32, its various components, and the functionalrelationship between the control unit 32 and various other elements ofthe trenchless underground boring system 12. The control unit 32includes a central processor 152 which accepts input data from thegeographic recording system 150, the PDU 28, and the GDAU 160. Thecentral processor 152 calculates the position of the boring tool 24 fromthe input data. The control processor 152 records the path taken by theboring tool 24 in the route recording database 154 and/or adds it to theexisting data in the route mapping database 158.

In an alternative embodiment, the central processor 152 also receivesinput data from the sensors 230 located at the boring tool 24 throughthe sensor input processor 232. In another embodiment, the centralprocessor 152 loads data corresponding to a predetermined path from theplanned path database 156 and compares the measured boring tool positionwith the planned position. The position of the boring tool 24 iscalculated by the central processor 152 from data supplied by the PDUinput processor 234 which accepts the data received from the PDU 28. Inan alternative embodiment, the central processor 152 also employs dataon the position of the PDU 28, supplied by the Geographic RecordingSystem 150, in order to produce a more accurate estimate of the boringtool location.

Corrections in the path of the boring tool 24 during a boring operationcan be calculated and implemented to return the boring tool 24 to apredetermined position or path. The central processor 152 controlsvarious aspects of the boring tool operation by use of a trenchlessground boring system control (GBSC) 236. The GBSC 236 sends controlsignals to boring control units which control the movement of the boringtool 24. These boring control units include the rotation control 238,which controls the rotating motor 19 for rotating the drill string 22,the thrust/pullback control 242, which controls the thrust/pullback pump20 used to drive the drill string 22 longitudinally into the borehole,and the direction control 246, which controls the direction activatormechanism 248 which steers the boring tool 24 in a desired direction.The PDU input processor 234 may also identify buried features, such asutilities, from data produced by the PDU 28. The central processor 152calculates a path for the boring tool 24 which avoids any possibility ofa collision with, and subsequent damage to, such buried features.

In FIGS. 28 and 29, there are illustrated flow charts for generalizedprocess and decision steps associated with boring a trenchless holethrough the ground. Initially, as shown in FIG. 28 and at step 350, anumber of ground-based transponders are positioned at appropriatelocations around a boring site. The trenchless underground boring system12 is then positioned at the appropriate initial location, as indicatedat step 352, and the transponders and geographic recording system areinitialized and calibrated, at step 354, prior to the commencement ofboring, at step 356. After boring has started, the PDU 28 probes theground at step 358 and then receives and analyzes the signature signalat step 360. Independent of, and occurring concurrently with, theprobing and receiving steps 358 and 360, the GRS receives position dataat step 362 and determines the position of the PDU 28 at step 364. Aftersteps 362 and 364 have been completed, the central processor 152 thendetermines the position of the boring tool 24 at step 366.

The central processor 152 then compares the measured position of theboring tool 24 with the expected position, at step 368, as given in theplanned path database 156 and calculates whether or not a correction isrequired to the boring tool direction, at step 370, and provides acorrection at step 372, if necessary. The trenchless underground boringsystem 12 continues to bore through the ground at step 374 until theboring operation is completed as indicated at steps 376 and 378. If,however, the boring operation is not complete, the central processor 152decides, at step 380, whether or not the PDU 28 should be moved in orderto improve the image of the boring tool 24. The PDU 28 is then moved ifnecessary at step 382 and the probing and GRS data reception steps 358and 362 recommence. The operation is halted after the boring tool 24 hasreached a final destination.

In an alternative embodiment, shown in dashed lines in FIGS. 28 and 29,the central processor 152 records, at step 384, the calculated positionof the boring tool 24 in the route mapping database 158 and/or the routerecording database 154, after determining the position of the boringtool at step 366. In another embodiment, the steps of comparing (step368) the position of the boring tool 24 with a pre-planned position andgenerating any necessary corrections (steps 370 and 372) are omitted asis illustrated by the dashed line 386.

It will, of course, be understood that various modifications andadditions can be made to the preferred embodiments discussed hereinabovewithout departing from the scope or spirit of the present invention.Accordingly, the scope of the present invention should not be limited bythe particular embodiments described above, but should be defined onlyby the claims set forth below and equivalents thereof.

What is claimed is:
 1. An apparatus for controlling an underground boring tool, comprising:a boring tool coupled to a drill pipe; a driving apparatus coupled to the drill pipe for driving the boring tool along an underground path; a detector interface that receives telemetry data from at least one of a plurality of antennae substantially in real-time, the telemetry data representative of a location of the boring tool along the underground path; and a control system comprising a controller communicatively coupled to the driving apparatus and the detector interface, the controller receiving the telemetry data from the detector interface and computing the location of the boring tool substantially in real-time, the controller transmitting control signals to the driving apparatus substantially in real-time to control one or both of a rate and a direction of boring tool movement along the underground path relative to the computed boring tool location.
 2. The apparatus of claim 1, wherein the driving apparatus comprises:a rotation unit coupled to a rotation unit control that moderates rotation unit performance; and a displacement unit coupled to a displacement unit control that moderates displacement unit performance, the drill pipe coupled to the rotation unit for rotating the boring tool and coupled to the displacement unit for displacing the boring tool along the underground path; wherein the controller transmits control signals to each of the rotation and displacement unit controls substantially in real-time to control one or both of the rate and the direction of boring tool movement along the underground path relative to the computed boring tool location.
 3. The apparatus of claim 1, wherein the controller computes the location of the boring tool in at least two of x-, y-, and z-plane coordinates using the telemetry data received by the detector interface.
 4. The apparatus of claim 1, wherein the controller computes an orientation of the boring tool in at least two of yaw, pitch, and roll using the telemetry data received by the detector interface.
 5. The apparatus of claim 1, wherein the telemetry data further comprises data representative of an orientation or a depth of the boring tool.
 6. The apparatus of claim 1, wherein the detector interface receives the telemetry data as an electromagnetic signal or an acoustic signal.
 7. The apparatus of claim 1, further comprising a beacon provided at the boring tool, the detector interface receiving telemetry data representative of the boring tool location derived from use of the beacon.
 8. The apparatus of claim 1, further comprising a beacon provided at the boring tool and a detector that detects a signal emitted by the beacon, the detector interface receiving telemetry data representative of the boring tool location from the detector.
 9. The apparatus of claim 1, further comprising a signal generator provided at the boring tool and a detector that receives a boring tool signal from the signal generator, the detector interface receiving telemetry data representative of the boring tool location from the detector.
 10. The apparatus of claim 1, further comprising:a probe signal generator, separate from the boring tool, that generates a probe signal; a signal generator provided at the boring tool that produces a signature signal in response to the probe signal, the signature signal having a characteristic that differs from the probe signal in one of timing, frequency content, information content, or polarization; and a detector that produces the telemetry data representative of the boring tool location, the detector interface receiving the telemetry data from the detector.
 11. The apparatus of claim 1, further comprising a ground penetrating radar (GPR) unit, the detector interface receiving the telemetry data from the GPR unit.
 12. The apparatus of claim 1, wherein the plurality of antennae comprises an array of antennae.
 13. The apparatus of claim 1, wherein the plurality of antennae comprises a plurality of spaced-apart antenna cells situated along the underground path.
 14. The apparatus of claim 1, further comprising one or more geophysical sensors for sensing one or more geophysical characteristics of earth along the underground path, wherein the controller modifies one or both of the rate and the direction of boring tool movement along the underground path in response to signals received from the geophysical sensors.
 15. The apparatus of claim 1, further comprising a display coupled to the controller for displaying a graphical representation of one or more of the boring tool location, orientation, the underground path, a buried object or boring tool movement along the underground path.
 16. The apparatus of claim 1, further comprising a pre-established bore plan representative of a desired underground path accessible by the controller, wherein:the controller compares the computed location of the boring tool with a desired boring tool location along the desired underground path and, in response to a difference between the computed and desired boring tool locations, transmits control signals to the driving apparatus substantially in real-time to control one or both of the rate and the direction of boring tool movement to urge the boring tool from the computed boring tool location to the desired boring tool location.
 17. A method of controlling an underground boring tool, comprising:rotating the boring tool; displacing the boring tool; detecting, by at least one of a plurality of antennae substantially in real-time, the boring tool while rotating and/or displacing the boring tool; computing, substantially in real-time, a location of the boring tool in response to detection of the boring tool; and controlling, substantially in real-time, one or both of a rate and a direction of boring tool movement along the underground path relative to the computed boring tool location.
 18. The method of claim 17, wherein detecting the boring tool further comprises detecting an electromagnetic signal or an acoustic signal emitted by the boring tool.
 19. The method of claim 17, wherein detecting the boring tool further comprises detecting a beacon signal from the boring tool, and computing the location of the boring tool further comprises computing the location of the boring tool using the detected beacon signal.
 20. The method of claim 17, further comprising:receiving a probe signal at the boring tool; producing a signature signal at the boring tool in response to the probe signal, the signature signal differing from the probe signal in one of timing, frequency content, information content, or polarization; and detecting the boring tool using the signature signal.
 21. The method of claim 17, wherein detecting the boring tool further comprises detecting, by at least one of he plurality of antennae, a signal emitted by the boring tool.
 22. The method of claim 17, wherein computing the location of the boring tool further comprises computing the boring tool location in at least two of x-, y-, and z-plane coordinates.
 23. The method of claim 17, further comprising computing an orientation or a depth of the boring tool substantially in real-time.
 24. The method of claim 17, further comprising computing an orientation of the boring tool in at least two of yaw, pitch, and roll.
 25. The method of claim 17, wherein controlling boring tool movement further comprises modifying one or both of the rate and the direction of boring tool movement along the underground path in response to geophysical information acquired for the underground path.
 26. The method of claim 17, further comprising displaying a graphical representation of one or more of a boring tool location, orientation, the underground path, a buried object or boring tool movement along the underground path.
 27. The method of claim 17, further comprising:providing a pre-established bore plan representative of a desired underground path; comparing the computed location of the boring tool with a desired boring tool location along the desired underground path; and in response to a difference between the computed and desired boring tool locations, controlling one or both of the rate and the direction of boring tool movement to urge the boring tool from the computed boring tool location to the desired boring tool location.
 28. An apparatus for controlling an underground boring tool, comprising:means for rotating the boring tool; means for displacing the boring tool; means for detecting, by at least one of a plurality of antennae substantially in real-time, the boring tool while rotating and/or displacing the boring tool; means for computing, substantially in real-time, a location of the boring tool in response to detection of the boring tool; and means for controlling, substantially in real-time, one or both of a rate and a direction of boring tool movement along the underground path relative to the computed boring tool location.
 29. The apparatus of claim 28, wherein the detecting means further comprises means for detecting an electromagnetic signal or an acoustic signal emitted by the boring tool.
 30. The apparatus of claim 28, wherein the detecting means further comprises means for detecting a beacon signal from the boring tool, and the computing means further comprises means for computing the location of the boring tool using the detected beacon signal.
 31. The apparatus of claim 28, further comprising:means for receiving a probe signal at the boring tool; means for producing a signature signal at the boring tool in response to the probe signal, the signature signal differing from the probe signal in one of timing, frequency content, information content, or polarization; and means for detecting the boring tool using the signature signal.
 32. The apparatus of claim 28, wherein the detecting means further comprises means for detecting, by at least one of the plurality of antennae, a signal emitted by the boring tool.
 33. The apparatus of claim 28, wherein the computing means further comprises means for computing the boring tool location in at least two of x-, y-, and z-plane coordinates.
 34. The apparatus of claim 28, further comprising means for computing an orientation or a depth of the boring tool substantially in real-time.
 35. The apparatus of claim 28, further comprising means for computing an orientation of the boring tool in at least two of yaw, pitch, and roll.
 36. The apparatus of claim 28, wherein the controlling means further comprises modifying one or both of the rate and the direction of boring tool movement along the underground path in response to geophysical information acquired for the underground path.
 37. The apparatus of claim 28, further comprising means for displaying a graphical representation of one or more of a boring tool location, orientation, the underground path, a buried object or boring tool movement along the underground path.
 38. The apparatus of claim 28, further comprising:means for providing a pre-established bore plan representative of a desired underground path; means for comparing the computed location of the boring tool with a desired boring tool location along the desired underground path; and means for controlling one or both of the rate and the direction of boring tool movement to urge the boring tool from the computed boring tool location to the desired boring tool location in response to a difference between the computed and desired boring tool locations. 